The enthalpies of formations of Ce2(MoO4)3(s) and Sm2(MoO4)3(s) have been measured at 298.15 K using semi adiabatic solution calorimetry. The precipitation reaction between RE(NO3)3.6H2O(s) (R= Ce, Sm) and ammonical solution of Na2MoO4(s) was studied. From the enthalpy of precipitation and other required auxiliary data, ΔfHom (298.15K) of Ce2(MoO4)3(s) and Sm2(MoO4)3(s) have been calculated for the first time as -4388.7 ± 3.6 and -4363.4 ± 4.1 kJ mol-1, respectively. The enthalpy of hydration of anhydrous Ce(NO3)3(s) to Ce(NO3)3.6H2O(s) has been calculated. ΔfHom(MoO42-, aq, 298.15K) has also been measured and calculated as -995.1 kJ mol-1 from required literature data.