Microcalorimetric measurements at elevated temperatures of the enthalpies of sublimation, bromination and iodination of CoMn(CO) 9, CoRe(CO) 9 and MnRe(CO) 10 have been made to obtain the standard enthalpies of formation Δ fH 0 (c) and Δ fH 0 (g) (in that order) as follows (values in kJ mol −1): CoMn(CO) 9 −(1460±36), −(1388±42); CoRe(CO) 9 −(1452±36), −(1369±42); MnRe(CO) 10 −(1630±24), −(1544±28). The values of Δ fH 0 m (g) are used to derive the enthalpy contributions of the metal–metal bonds in these molecules D(CoMn)=(114±23), D(CoRe)=(122±25) and D(MnRe)=(145±29) kJ mol −1. In each case, the value of D(M–M′) is less than the mean of the corresponding homometallic bond enthalpy contributions.