A major feature of the crystal structure of K[AsMe 2 S 2 ] . 2H 2 O, which contains two independent formula units, is the presence of infinite sheets formed by edge sharing between two different types of K(H 2 O) 4 S 2 octahedra in which the sulfurs occupy either cis or trans positions. Although the four independent arsenic-sulfur distances all fall between those for single and double bonds, one sulfur at each arsenic is coordinated to potassium and the As-S distance here is longer [mean 2.140(2) 9] than that to the second sulfur [mean 2.111(5) 9] which is effectively free. The structure also contains a series of weak O-H . . . S hydrogen bonds.