To understand the excited state dynamics as a function of Cs2SnBr6/g-C3N4 composition, Differential Transmission (DT) measurements were performed. Excitation of solutions in quartz cuvettes was provided by a femtosecond pulsed laser at 350 nm, and the transient transmission signal was collected in a pump and probe configuration with sub-ps resolution (details about experimental method in SI). A set of data extracted from the measurements are shown in Fig. , where DT spectra are reported for the different compositions, from 100% g-C3N4 (black line) to 100% Cs2SnBr6 (green line). By observing the two photobleaching features centered at 500 nm and 630 nm, it is worth noticing that the intensity of transitions changes substantially with the change in composition. First of all, lower overall DT signal amplitude is obtained from the pure compounds, which are also the ones with lower ammonia evolution rates. When adding small amounts of perovskite to the g-C3N4 the intensity of the signal is boosted, analogously to what already observed for the ammonia production rate in Fig. and, correspondingly, a broad DT peak centered around 630 nm rises. When the perovskite amount surpasses 40%, the amplitude of the DT signal decreases down to the pure perovskite case. Another interesting piece of information that can be extracted from DT measurements is the fact that DT spectra of composites with increased evolution rates (particularly from 7% to 35% perovskite content) have the same spectral features of pure g-C3N4, suggesting that the reaction occurs mainly at the g-C3N4 defect sites, with an introduction of an amount of perovskite <40% favoring the charge transfer towards g-C3N4 sites. The time evolution of the signal is reported in Figure showing that, after a first initial decay on a picosecond scale, the signal stabilizes and follows a much slower decay. To get a complete overview of the data, in Fig. are reported the integrated DT signals for different wavelength ranges and compositions. A fairly good agreement is found between the trend of the DT signals and that of the ammonia generation reported in Figure . Time resolved photoluminescence emitted by the composites was also investigated and the results are reported in Figure : here the time decays show that compounds with low-intermediate perovskite amounts, corresponding to high ammonia production rates, have longer TRPL lifetimes. This suggests that for these compositions there is some degree of suppression of nonradiative decays for photoexcited carriers, with a lifetime longer than that of both pure Cs2SnBr6 and pure g-C3N4. To analyze the NH3 production mechanism, we started by investigating the pure vacancyordered double perovskite phase. As we can see from the experimental ammonia production rate in Figure , this system, at 100% of perovskite, already generates a significant amount of ammonia.