The densities of the melts in the systems MF-M2SO4 (where M is Li, Na, K) were measured using the method of hydrostatic weighing. From the volume properties point of view the investigated systems exhibit almost ideal behaviour which indicates that the congruently melting compounds Na3FSO4 and K3FSO4, which are formed in the systems NaF-Na2SO4 and KF-K2SO4, respectively, exhibit a low thermal stability and at melting they undergo a considerable thermal dissociation. The degree of the thermal dissociation was calculated on the basis of both the thermodynamic analysis of the phase diagrams and the volume properties of the investigated systems. It was found that the degree of thermal dissociation of Na3FSO4 at the melting point attains the value alpha0 = 0.71 while that of K3FSO4 is alpha0 = 0.78. The dissociation enthalpies calculated on the basis of the density values are DELTA(dis)H(Na3FSO4) = 40.2 kJ mol-1 and DELTA(dis)H(K3FSO4) = 24.1 kJ mol-1.