For the very first time, sulfonium polyoxometalate (POM) assemblies are shown to develop efficient solid-state photochromism in ambient conditions. The optical properties of the already known Rb-0.75(NH4)(5.25)[(Mo3O8)(2)O(O3PC(CH2S(CH3)(2))OPO3)(2)]center dot 8H(2)O (1) and a new material (Me3S)(4)[Mo8O26] (2) under UV excitation are investigated by diffuse reflectance spectroscopy, revealing that the color change effect is highly tunable playing with the nature of the POM. A mechanism involving the photoreduction of Mo6+ cations associated with electron transfers from the sulfonium cations toward the POMs is proposed.