In recent years, several type II and Z-scheme heterojunction including MHPs have been reported for a range of photocatalytic reactions. The partner semiconductor of choice, in most cases, has been graphitic carbon nitride (g-C3N4). g-C3N4 is one of the most appealing and performing metal-free photocatalysts having a good absorption in the visible spectrum (band gap of about 2.7 eV), ease of preparation from cheap precursors, it is atoxic, and is stable in both acidic and basic media. In MHPs/g-C3N4 heterojunctions, photogenerated electrons tend to migrate and accumulate in g-C3N4, where active sites for the photocatalytic reaction are present, while holes from graphitic carbon nitride migrate to the MHP. On the other hand, the perovskite itself can efficiently photogenerate hydrogen by hydrohalic acid splitting, as demonstrated in MAPbI3 (MA=methylammonium) and then in other systems such as CsPbBr3. Moving away from toxic lead, Bi-based perovskite derivatives were also successfully employed in heterojunctions for hydrogen photogeneration. More recently, significant interest has been triggered by Bi-based double perovskites for their potential use in different photocatalytic reactions, thanks to their superior stability, wide compositional tunability, and good optical properties. Cs2AgBiBr6 has been investigated for photocatalytic CO2 reduction, where it has been found that the surface of the perovskites acts as reaction site in the reduction process. Fewer reports refer to the hydrogen generation mediated by this double perovskites, with the NiCoP/Cs2AgBiBr6 heterojunction representing the best report to date with about 400 μmol g -1 h -1 of evolved hydrogen under 300 W illumination at λ ≥ 420 nm. The proposed mechanism in this system considers an effective transfer of electrons from Cs2AgBiBr6 to the CB of NiCoP on which the H2 generation reaction occurs. While there is an increasing amount of evidence about the effective role of MHPs and MHP-based heterojunctions in running various photocatalytic processes, a detailed description of the reaction mechanism(s) is still lacking. For this reason, herein, we report a combined experimental and computational study on Cs2AgBiCl6/g-C3N4 heterojunctions which has been applied to run two different photocatalytic reactions: hydrogen generation and nitrogen reduction to ammonia, to the best of our knowledge. This last reaction has never been explored before in any MHP-based system. Cs2AgBiCl6 has been selected based on its optimal moisture stability, suitable band alignment and because it is a lead-free compound. The Cs2AgBiCl6/g-C3N4 composite system has been investigated in the whole weight ratio between perovskite and g-C3N4. We found that MHP-rich compositions perform better in the H2 photogeneration, while g-C3N4-rich systems are more effective in the nitrogen photofixation reaction. As reported above, due to the limited microscopic details about the photocatalytic activity of MHP-based heterojunctions, key aim of this work is to provide a comprehensive description of the mechanism underpinning the solar-light mediated reactions in terms of charge carrier dynamics and reaction active sites. This has been achieved through a detailed photophysical and computational modelling study on the prepared systems and for the two photocatalytic reactions investigated providing solid clues about reaction mechanisms. While the application of MHPsbased heterojunction for hydrogen photogeneration is now quite well established, the present work clarifies the suitability of such composite systems to run nitrogen reduction reaction, exploring a new avenue in the photocatalytic application of MHPs.