The densities and surface tensions of the binary molten systems M2MoO4¿MoO3 and M2WO4¿WO3 (M = Li, Na, K) have been measured by the method of the buoyancy exerted on a Pt bob and by the ring method respectively. From density and surface tension values molar volumes and molar free surface energies were calculated. The isotherms of all these properties show characteristic differences between molybdate and tungstate systems. The trends of these isotherms give an indication of the occurrence of large complexes in the molten molybdates, including Mo6+ ions with co-ordination numbers higher than 4. In the molten tungstates probably only complexes exist of small average dimension, consisting of chains of WO4-tetrahedra. The proposed structures agree with those assumed by Gelsing et al. and Van der Wielen et al. for the vitreous alkali tungstates and molybdates respectively.