The specific heat, heat flux (DTA trace) and dielectric constant of KDP ferroelectric crystalhave been measured simultaneously for various electric fields with a conduction calorimeter.The specific heat presents a strong anomaly, but these simultaneous measurements allow usto evaluate the latent heat accurately. The latent heat decreases with field, and the value ofthe critical electric field—that where the latent heat disappears—is estimated to be(0.44 ± 0.03) kV cm−1. Incidentally, we have measured simultaneously the dielectric permittivity, which suggeststhat latent heat is developed as domains are growing.