As a general observation, the combination between g-C3N4 and the perovskite enhances the DT signal of the component actively involved in the reaction. Measurements were repeated for different compositions, and the maximum value of the DT signal was extracted for the two different features of the spectrum, i.e., the one at 500-520 nm and the one at 630-650 nm, as reported in Figure . In the case of composites optimized for HER, the spectrum mainly consists of photoinduced absorption (negative DT, thus positive differential absorption DA=-DT) and evolves on a picosecond timescale, as shown in Figure . Interestingly, the trend of the DA signal as a function of composition was closely related to the trend of HER efficiency. In the case of composites optimized for ammonia generation, a relevant correspondence was found instead between the NH3 production efficiency and the peak of DT signal at long time delay (above 1ns), as shown in Figure . This observation confirms that for NH3 production photoinjected carriers, which are always generated in the perovskite, are transferred to the g-C3N4, where the ammonia evolution reaction occurs, within a nanoseconds timescale. Additional relevant data reporting the whole DT decay for different compounds and spectra extracted at longer time delay can be found in Supporting Information (Figures and).