The crystal and molecular structures of two thiosemicarbazones have been determined by X-ray diffraction techniques. The antitumor agent 5-hydroxy-2-formylpyridine thiosemicarbazone sesquihydrate forms pale-yellow, orthorhombic crystals, with a= 13.261 (10), b=11.159 (5) and c= 13.699 (4) ,~, space group Pbcn, Z = 8, 0(calc) = 1.466 g cm-3 and 0(obs) = 1 "46 g cm -3. Crystals of acetonethiosemicarbazone are colorless, with a=7.854 (4), b=7.791 (4), c= 6.307 (2) ,~, c~= 111.52 (2), fl= 109.82 (2) and y=85.12 (2) °, space group PI indicated by the intensity statistics. With two molecules per unit cell 0(calc)= 1.282 g cm -3 compared to o(obs)= 1.27 g cm -3. In both molecules the sulfur atom is trans to the nitrogen atom relative to the C-N bond. The most significant feature is the longer N-N bond [1-398 (6) ,~] in acetonethiosemicarbazone compared to the N-N bond [1.379 (6) A] in the 5-hydroxy-2-formylpyridine thiosemicarbazone. The shorter bond length appears to be related to an interaction with the pyridine ring.