Group IIIA metal Al3+/Ga3+-capped {M2(H2O)[P4Mo6O31]2}-based phosphomolybdates were synthesized by adjusting the reducing agents, in which two [P4MoVI4MoV2O31]8- half-units are joined in a face-to-face mode and are capped by two M3+/[M(H2O)]3+ cations on both sides. This type of {M2(H2O)[P4Mo6O31]2} hybrids showed excellent photocatalytic activities in the Cr(VI) reduction reaction.