Two new alkali metal chromium thiophosphates, Na 3 Cr 2 P 3 S 12 and K 3 Cr 2 P 3 S 12 , have been synthesized and their structure determined from single-crystal or powder X-ray di4raction analyses. These isostructural compounds, which exhibit a novel structural arrangement, crystallize in the monoclinic system (space group P2 1 /c) with the cell parameters a β«Ψβ¬ 17.4076( 10) A s , b β«Ψβ¬ 11.1723(10) A s , c β«Ψβ¬ 19.2502(11) A s , β«Ψβ¬ 149.731(3)3 (V β«Ψβ¬ 1887.1(2) A s 3 , Z β«Ψβ¬ 4) and a β«Ψβ¬ 17.9690(14) A s , b β«Ψβ¬ 12.0607(5) A s , c β«Ψβ¬ 19.3109(16) A s , β«Ψβ¬ 150.008(3)3 (V β«Ψβ¬ 2091.98( 16) A s 3 , Z β«Ψβ¬ 4) for Na 3 Cr 2 P 3 S 12 (I) and K 3 Cr 2 P 3 S 12 (II), respectively. For (I), the single-crystal re5nement led to R/R w (F 2 ) β«Ψβ¬ 0.0417/0.0650 (for 4125 independent re6ections and 182 re5ned parameters). For (II), the Rietveld re5nement led to R p /R wp β«Ψβ¬ 0.0181/0.0233 (for 90 re5ned parameters). In both materials, the structures consist of 1 [Cr 2 P 3 S 12 ] 3 Ψ chains built upon two edge-sharing [CrS 6 ] octahedra capped by two [PS 4 ] tetrahedra de5ning isolated [Cr 2 P 2 S 12 ] Ψ 8 entities. These entities are linked to each other through an extra [PS 4 ] tetrahedral group. Magnetic measurements evidence antiferromagnetic coupling between Cr 3 Ψ cations. The charge balance of the phase is