The stability constant of Co[EtO)2PS2]2.2py was obtained by a spectrophotometric method with K = 0.66 ?0.05 mol dm-3. Three crystals with different colors, namely violet, pink and colorless, were separated Difference in structure of the violet and pink crystals were indicated by spectrophotometry and TG-DTA. The colorless crystal is a dimer. Mechanism of reductive reaction was suggested. The structure of the violet crystal was determine by x-ray diffraction anal. The crystal belongs to monoclinic system, a 8.117(7), b 17.813(4), c 9.901(3) ? b 104.19(5)? d = 1.406 g cm-3, Z = 2, space group P21/c.

Key words: CRYSTAL STRUCTURE DETERMINATION, PYRIDINE P, X-RAY DIFFRACTION ANALYSIS, SUBSTITUTION REACTION, ETHOXY GROUP, EQUILIBRIUM CONSTANT, THERMOGRAVIMETRY, COBALT COMPLEX, REDUCTION REACTION, DIFFERENTIAL THERMAL ANALYSIS