SummaryThe thermal diffusivity has been measured for 10 glass ionomer and resin‐based materials: three conventional (water‐hardened) glass ionomer cements, two silverreinforced glass ionomers, an experimental stainless steel‐reinforced glass ionomer, three visible light‐cured (VLC) glass ionomer‐resin hybrid materials, and a VLC resin‐based product developed for the same clinical uses as the hybrid materials. Cube‐shaped specimens, c. 10 × 10 × 10 mm, initially at room temperature were immersed in mercury surrounded by an ice‐water bath. From the experimental cooling curve a semi‐log plot of relative temperature decrease vs. time yielded a straight line whose slope is proportional to the thermal diffusivity. The values ranged from 1.74–5.16 × 10−3cm2s−1, and all of the materials tested would have adequate insulating properties provided normal clinical thickness levels for lining materials are maintained. It was found that the thermal diffusivities for the three metal‐reinforced glass ionomers, where composition information is avaiable, do not follow a rule of mixtures applied