Cs2AgBiCl6/g-C3N4 heterojunctions for hydrogen photogeneration and nitrogen reduction to ammonia have been investigated through advanced spectroscopy and computational modelling, aiming at unveiling the mechanism of these reactions. TAS provided detailed information on the charge carried dynamics in the heterojunction indicating that for high perovskite amounts (best performing compositions for H2 photogeneration) the optical excitations are confined on the Cs2AgBiCl6, where the reaction occurs. The opposite situation is found for low Cs2AgBiCl6 (high g-C3N4) containing systems (best performing compositions for nitrogen reduction) where the typical DT signal of g-C3N4 results to be enchanted. This finding suggests that active sites for ammonia production are confined in the carbon nitride.