Tachikawa et al. performed direct ab-initio molecular dynamics (AIMD) and suggested that the FA dimer cation could form a complex, where the carbonyl oxygen atom was bound symmetrically leading to the formation of a face-to-face complex. However, with increasing temperature, a proton transferred H + (HCOOH)-HCO 2 radical cation would be dominant with a 78% formation probability at room temperature, and absent at 0 K. Very recently, Sutton et al., using ultrafast time-resolved pump-probe mass spectrometry, monitored the proton transfer pathway following Rydberg state excitation for a series of FA clusters. A contact ion-pair mechanism was shown to be operational leading finally to a protonated FA cluster and the HCOO -anion. This pathway was absent in the FA dimer, a pathway purported to be operational upon VUV excitation, but whose efficiency increased with the number of FA clusters bound in the system. Recently, intra-cluster charge migration upon hydration of protonated FA was studied using both cold ion vibrational spectroscopy coupled with path integral ab-initio molecular dynamics simulations. The authors suggested that FA could be considered formally protonated when being hydrated by a maximum of three water molecules. Beyond that, as the water cluster number increases, the proton transfer occurs and the excess proton is accommodated by the extended water network and the neutral FA. There has also been a study recently where photoionization and electron impact ionization of FA and methanol co-deposited together on cold argon clusters led to interesting mixed cluster proton transfer reactions. In this work, we perform a systematic study combining theory and experiment in unrav-elling the photoionization dynamics of mixed FA water clusters. We present a synchrotronbased VUV photoionization 36 study of the mixed clusters formed in a supersonic expansion coupled with electronic structure calculations, where both E and Z conformers of FA are taken into account, to understand the photoionization dynamics. The employed methodology includes intensity measurements of various masses at variable photon energies, the appearance energies of various fragments, and an interpretation of their stability guided by theoretical calculations. This approach, previously used in studying proton transfer in acetaldehyde water clusters and naphthalene water clusters, 38 also contributed to identifying magic numbers in water clusters beyond the traditional "21" 39 and the identification of the hydrogen bond networks in diol and polyol water clusters. Mass spectra results demonstrate that upon ionization of mixed FA-water clusters, the resulting clusters are protonated. The stability of magic numbers of different FA-water clusters was observed in the mass spectra, specifically for clusters: