The title compound, C(21)H(21)O(2)PS(2), was obtained from the corresponding dilithio methandiide by treatment with iodo-methane. The bond lengths and angles deviate considerably from those in the dimetallated compound. These differences are most pronounced in the PCS backbone. While the title compound features C-P and C-S distances of 1.9082 (17) and 1.8348 (17) Å, respectively, the dianion showed C-P(av) distances shortened by 11% [1.710 (4) Å] and C-S distances shortened by 12% [1.614 (3) Å]. Additionally, the P-C-S angle experiences a contraction by methyl-ation of the dianion from 121.4 (2) to 111.96 (9)° in the title compound.