In order to understand the time-resolved T-jump data it is useful to first assign the features present in the steady-state FTIR spectra. The IR spectrum of hydrated ZSM-5 is well studied and Figure illustrates how the spectrum of the partially deuterium-exchanged zeolite changes across the temperature range investigated. The sharp feature at 2760 cm -1 corresponds to ν(SiOD) modes of silanol groups at the crystal exterior and in internal defects. It is present at all temperatures. The equivalent ν(SiOH) mode is visible at 3750 cm -1 . Above 125 °C, the free ν(OH) Brønsted acid site band is visible at 3590 cm -1 , and the equivalent ν(OD) band at 2655 cm We refer to these bands as ν(ZOH) and ν(ZOD) respectively. Hydrogen bonding of ZOH with one or two adsorbed water molecules results in a substantial broadening, structuring and red shifting of the ν(ZOH) band. This creates a broad doublet peaking at ca. 2900 cm -1 and ca. 2500 cm -1 . We refer to this band as ν (ZOH..(OH2)n=1,2) but note that only the 2900 cm -1 component of it is fully visible within the spectral range examined in the time-resolved experiments. The corresponding ν(ZOD..(OH2)n=1,2) band or bands are obscured by the zeolite lattice bands below 2100 cm -1 . Below 125 °C the spectrum shifts in shape towards a less structured form associated with water clusters observed for zeolites with > 2 H2O molecules per Brønsted acid site, referred to as ZO -…H + (H2O)n>2. Note also that at higher water coverages (lower temperatures) ν(OH)/ν(OD) bands due to weakly hydrogen bonded (terminal) OH/OD groups in hydrogen bonded clusters occur at frequencies close to those of ZOH and ZOD. The weak peak at 2705 cm -1 at high temperatures is due to a small population of deuterium exchanged extra-framework aluminium hydroxide species within the pore network, a feature common in commercial zeolite materials. Figure shows FTIR difference spectra centred on the timeresolved experiment region of hydrated ZSM-5 on heating from initial temperatures of 100 °C (a) and 175 °C (b). The spectra were collected as a continuous temperature ramp (2 °C per minute). We define the temperature difference T = Tramp -Tinit. Removal of Brønsted site-associated hydrogen-bonded water results in negative intensity changes in the difference spectra in this region. At small ΔT values for the 100 °C TInit sample it is mostly clustered water ZO -…H + (H2O)n>2 which is removed, giving a largely flat difference spectrum (Figure ). With increasing ΔT, at Tinit =100 °C, the removal of ZOH..(OH2)n=1,2 water molecules hydrogen-bonded to the protonated Brønsted sites results in broad negative features at ~ 2900 and 2500 cm -1 in the difference spectra due to loss of the hydrogen bonds. Superimposed on these broad negative features is a positive feature at 2660 cm -1 due to Brønsted acid ZOD groups recovered by the water loss. Note that the shape of this feature is distorted by the temperature dependent frequency shift well known for zeolite hydroxyl groups. There is also a negative feature at the SiOD frequency (2760 cm -1 ) which increases with increasing temperature. This is due to the fact that the absorption coefficient for ν(SiOD) vibrations decreases markedly with increasing temperature, as discussed further below. For TInit = 175 °C the sample contains no clustered water ZO -…H + (H2O)n>2 and the removal of ZOH..(OH2)n=1,2 water proceeds for all values of ΔT (Figure The spectral changes observed in the T-Jump transient spectra of Figure are consistent with the sample undergoing rapid heating due to absorption of the pump laser light, and qualitatively match the form of difference spectra taken by FT-IR for increasing temperature in Figure . For the sample held initially at 100 °C (Figure ) the loss of adsorbed water is seen as a negative baseline shift due to loss of water clusters, and the growth of two negative features at ~2900 cm -1 and below 2500 cm -1 due to loss of hydrogen bonded ZOD groups, as discussed above. These negative changes are ~ 5x smaller than those seen in the FTIR difference spectra, presumably because in the Tjump experiments the sample begins cooling before desorption reaches equilibrium. The subsequent decline of these negative features indicates that water readsorbs on a time scale of tens of milliseconds. The negative ν(SiOD) band indicates a temperature increase due to the pump pulse which can be quantified after appropriate calibration, as discussed below. The recovery of ZOD groups accompanying loss of hydrogen bonded water which is very evident in the FTIR temperature ramp difference spectra in Figure (frequency shift) components of the ν(OD) band for initial sample temperatures of 175 and 300 °C respectively. At 175 °C the two components decay with different time constants. At 300 °C, where no adsorbed water is present, the only contribution to the difference spectrum is the temperature dependent frequency shift, and the two components decay synchronously.