The thermal degradation of Zinc-O, O-diethyl dithiophosphate has been invesgitaged at 190?C in n-hexadecane. The major products have been identified as O, S, S-triethyl trithiophosphate (Ets)2,P(S) Oet, O-O,S-triethyl dithiphosphate, Ets P (S) (Oet),2, S,S,S-triehyl detrathiophosphate, (Ets)3 PS-O,S-S, triethyl dithiophosphate, (EtS)3 PS, O,S,S-triethyl dithiophosphate, (EtS) P(O) Oet and O,)s, trietyl thiophosphate, Ets P (O) (Oet)2. The degradation is suggested to involve thino-thiolo isomerisation coupled with disproportination of sulfur and oxygen linked to phosphorous.