Enthalpies of mixing in CoxMg1-xO solid solutions at T = 298 K have been determined using high temperature drop solution calorimetry in molten sodium molybdate (3Na(2)O . 4MoO(3)) solvent at T = 973 K. Slightly positive enthalpies of mixing were observed, which conform to a regular solution model with interaction parameter W = (5.1 +/- 0.3) kJ . mol(-1), which is similar to the interaction parameter calculated from previously measured activity-composition relations. Standard molar entropies S-m(0) at T = 298 K calculated from subambient C-p data for CoO, MgO are (52.8 +/- 0.1) J . K-1 . mol(-1) and (28.1 +/- 0.1) J . K-1 . mol(-1), respectively. Integration of the C-p,C-m/T of the Co0.50Mg0.50O mixture yields an entropy of (40.3 +/- 0.1) J . K-1 . mol(-1). This value does not include configurational entropy from the Co/Mg site disorder. The excess entropy of Co0.50Mg0.50O is zero within experimental error. Heat capacities were also measured between T = 298 K and T = 973 K. The enthalpies of mixing do not depend on temperature in this range. (C) 2003 Elsevier Science Ltd. All rights reserved.