The Bureau of Mines investigated the standard Gibbs energies of formation, agf deg., for MnF2 (manganese difluoride) and CoF2 (cobalt difluoride). High-temperature galvanic cells employing CaF2 (calcium difluoride) as the solid electrolyte were used to measure the open-circuit potentials for the cell reactions mn(c) + nif2(c) = mnf2(c) + ni(c) co(c) + nif2(c) = cof2(c) + ni(c). Combination of the standard Gibbs energy changes for these reactions with the standard Gibbs energy of formation of nif2 yielded the following: agf deg. (Mnf2) = (-203,008 + 30.96T)+/-560(745.7-1,078.3 K) and agf deg. (Cof2) = (-159,090 + 32.13T)+/-420 (769.5-1,026.8 K), where agf deg. is expressed in calories per mol (1 cal = 4.184J) and T is expressed in kelvins. The standard enthalpies of formation ahf deg., Derived by the third-law method, are ahf deg. 298(Mnf2) = -204, 633+/-560 cal/mol and ahf deg. 298(Cof2) = -161,166+/-420 cal/mol.