The heat capacities of the isostructural dihydrated formates of Mn2+, Fe2+, and Ni2+have been measured between 1.4° and 20°K. Sharp peaks associated with long‐range spin ordering are found at 3.72°K for Mn(HCOO)2·2H2O, 3.74°K for Fe(HCOO)2·2H2O, and 14.5°K for Ni(HCOO)2·2H2O. An additional sharp anomaly inCpis found at 1.72°K for the Mn+2salt, whileCpof the Ni+2salt exhibits a Schottky‐like maximum at 3°K. Each of these anomalies can be correlated with some feature of the observed magnetic susceptibilities.A unit cell in these salts contains two each of two inequivalent metal ion sites, types A and B. Formate groups constitute AA and AB bridges and appear to mediate the dominant superexchange couplings. A simplified molecular field treatment of Mn(HCOO)2·2H2O in whichJAA= 0.65k,JAB= −0.10k, andJBB= 0 provides interpretations of several striking effects. In particular, it shows that below a common transition temperature, B spins order much more slowly than A spins and are nearly ``free'' at 1°K as seen in both magnetic and thermal data. Analysis of the data for the Ni+2and Fe2+salts suggests that the effective spins of A and B ions may differ in a given substance.