We have carried out ac ombined experimental and theoretical investigation of the structures and properties of af amily of cadmium phosphochlorides with varying Cl/Cd and P/Cd ratios,C d 2 P 3 Cl, Cd 4 P 2 Cl 3 ,C d 3 PCl 3, and Cd 7 P 4 Cl 6 . Their optical band gaps are in the visible region and the values are sensitive to the Cl/Cd and P/Cd ratios,l eading to an increase and decrease,r espectively.F irst-principles calculations were used to understand the bonding and electronic structures.All phosphochlorides except Cd 2 P 3 Cl possess direct band gaps.T he calculated dielectric constants and Born effective charges illustrate the bonding,h ybridization, and ionic character in these compounds.T he band positions indicate the thermodynamic feasibility to perform water splitting.A ll systems can be used in the hydrogen evolution reaction (HER), where Cd 7 P 4 Cl 6 has the highest activity and Cd 3 PCl 3 the lowest. The apparent quantum yield is highest in Cd 7 P 4 Cl 6 (20.1 %) even without the assistance of aco-catalyst. The HER activity can be understood on the basis of photoelectrochemical measurements.