The combustion enthalpies of Mo6S8-y clusters (7.8 less than or equal to 8 - y less than or equal to 8.0) were measured at 773 K in flowing oxygen using a Calvet-type twin calorimeter. There was a maximum value at the sulfur composition 8 - y = 7.9. Evaporated gas ratios of SO3/SO2 for each sulfur composition were calculated using the results of quantitative chemical analysis of SO,. From the data of the combustion enthalpy combined with the values of the formation enthalpies of MoO3, SO2 and SO3, the formation enthalpy of Mo6S8-y clusters was determined. The standard formation enthalpies of Mo6S8-y clusters were also determined by measuring the heat content for clusters with various compositions in the temperature range from 298 to 773 K.