Thermal expansion of some members of the synthetic solid solution (a) over circle kermanite-gehlenite was measured (in the range 25-1200degreesC) for the very first time, with the aim of clarifying the behaviour of this solid solution both with respect to composition and temperature. The results confirmed the non-ideal behaviour at room temperature (negative excess molar volume), and showed a different non-ideal response at high temperature. In fact, excess molar volume is different during the heating and cooling stages: for X-ak<0.5 it becomes, from almost ideal, strongly positive at high temperature, while for X-ak>0.5 it is always negative. It can then be inferred that (i) lattice dimensions are very sensitive to cation diffusion activated by temperature, (ii) intra-crystalline partition mechanisms, and their effects on the structural features, vary as a function of composition in the solid solution studied.