The molar volume of IVb-chalcogenide systems has been reviewed. Anomalies in the composition and temperature dependence of the molar volume of the melt can be attributed to associates corresponding to an equi-atomic compound and a 1:2 compound, respectively, having ionic and covalent character. The latter becomes unstable with rising temperature, which causes anomalous contraction of the volume. The instability may be related with the Peierls distortion mechanism in the p-bonded systems.