High-temperature X-ray diffraction experi-ments have been performed on melilite solid solutions, on single crystal and powder samples. In åkermanite–gehle-nite series, the volumetric thermal expansion increases from gehlenite, 26.5(2) E-06 K-1, to åkermanite, 31.2(1) E-06 K-1. The variation is related to the cation content in the tetrahedral T1 site of the structure, and the linear expansion along the a axis has the greatest variation as a function of composition. The expansion perpendicular to the tetrahedral layers does not present any significant variation as a function of composition. In the åkermanite– Na melilite series, the presence of Na in the [8]-coordi-nated site strongly increases the linear expansion along the c axis, while the volumetric expansion decreases from åkermanite to Na melilite, whose value can be extrapolated to 27.4 E-06 K-1