The crystal structures of the title compounds P,P-diphenyl-N-phenyl-phosphinylthioformamide ([Ph2P(=O)C(=S)N(H)Ph], LOH) and P,P-diphenyl-N-phenyl-thiophosphinylthioformamide ([Ph2P(=S)C(= S)N(H)Ph], LSH) have been determined at room temperature. Bright yellow crystals of LOH are triclinic, space group P[unk] with unit cell dimensions a = 8.785(1) Å, b = 12.004(2) Å, c = 8.741(1) Å, α = 110.06(1)°, β = 94.63(1)°, γ = 100.82(1)°, Z = 2 and Dx = 1.334 Mg m−3. Yellow crystals of LSH are orthorhombic, crystallizing in the Pbca space group with dimensions a = 11.922(1) Å, b = 8.742(1) Å, c = 34.043(2) Å, Z = 8 and Dx = 1.323 Mg m−3. The structures were solved by direct methods and each refined by a full-matrix least-squares procedure to final R = 0.051 using 2059 reflections for LOH and R = 0.042 using 1325 reflections for LSH. The molecular structures indicate the presence of relatively weak P – C bonds for the central chromophores. This, plus the lack of planarity of the P(Y)C(S)N(H), Y = O or S, moieties suggests little delocalization of π-electron density over the respective chromophores. The structure of LSH is strictly molecular whereas molecules of LOH associate via hydrogen bonds linking the amide H atoms and the O atoms to form centrosymmetric dimers.