The synthesis of platinum carbonyl clusters of a [Pt3(CO)6]n 2-formula, known as Chini clusters, was proposed in the 1960s . They can be synthesized by chemical reduction by CO in alkaline alcoholic solution. They can also be synthesized by radiolysis in alcohol solution under CO atmosphere, the nucleation n can be tuned by the irradiation dose . They consist of multiple stacking units of Pt3 triangles stabilized by CO ligands bound in two diferent ways: terminal (connected to one Pt atom) and bridging (connected to two Pt atoms) -see Figure . Their physicochemical properties and their optical properties depend on the nuclearity number n. Their redox properties have been studied by pulse radiolysis . They serve as precursors for Pt nanoparticles and nanowires, substrates to obtain metal-organic frameworks , or even alloy nanoclusters if mixed with other metals . The Chini clusters can be deposited on diferent supports to be used as composite materials in several electrocatalytic application and photo-and thermo-catalytic processes . function, extremely low required overpotential, and optimal hydrogen adsorption-free energy . Most of the studies in this topic report on the use of Pt-based nanoparticles for photocatalytic hydrogen formation, however articles on clusters of controlled size as cocatalysts are scarce. Here we aim to check the applicability of carbonyl Pt clusters in photocatalytic hydrogen generation by determining their electronic, photochemical, and charge transport properties as a function of their nuclearity. By assuming that the reaction involves the formation of H2 via the recombination of two hydrogen atoms, we focus on the binding of an atomic hydrogen and a H2 molecule by the Chini clusters of n = 1-8. Our theoretical predictions are followed by experimental proof of concept, in which synthesized Chini clusters are deposited on TiO2 and such a composite is tested in photocatalytic hydrogen generation.