Heats of transition among the Li 2WO 4 polymorphs, Li 2WO 4I (phenacite-type structure), Li 2WO 4II, Li 2 WO 4III, and Li 2WO 4IV, and that between Li 2MoO 4 (phenacite) and Li 2MoO 4(spinel) were measured by transposed temperature drop calorimetry. The heats of fusion of Li 2WO 4I and Li 2MoO 4(ph) were also obtained. Using these data, the phase boundaries among the polymorphs of Li 2WO 4 and of Li 2MoO 4 were calculated. The calculated phase diagrams were compared with those reported previously. They agree well for Li 2WO 4 but show significant discrepancies, perhaps related to problems in attaining equilibrium at lower temperature, for Li 2MoO 4.