Differential thermal analysis has been performed on barium sodium niobate single crystals. A thermal effect is detected for the first time at the 260°C ferroelastic transition. A large thermal peak is also observed at the 570°C Curie point. For both transitions an estimate of the latent heat is obtained by comparison with thermal effects recorded in gadolinium molybdate and quartz single crystals. The results for the first‐order 570°C transition is in good agreement with existing dielectric data. First order is also assigned to the lower ferroelastic transition whose features are discussed in the framework of the available thermodynamic theory.