The thermal expansions of trimerite and beryllonite have been determined up to 800°C. No anomalies were observed in the thermal expansion curves but the expansion coefficients for beryllonite are approximately twice those of trimerite. The thermal expansion behaviour of the minerals is interpreted in terms of a tetrahedral tilting model with the main change in apparent tilting being about the monoclinic 010 plane, i.e. the 0001 pseudo-hexagonal plane.