The coefficients in the equation of state of BaTiO3 proposed by Devonshire can be considered as describing the polarization and temperature dependence of the dielectric constant. This dependence has been studied by a dynamic method. The polarization is developed by a large-amplitude at field and the dielectric constant is measured by means of a small rf signal. This method provides an accurate measurement of the coefficient of the P4 term in Devonshire's equation for the free energy of BaTiO3. This coefficient has been studied as a function of temperature, and, contrary to previous assumptions, has been found to have a strong temperature dependence, decreasing linearly, with temperature. The fields which can be applied without breaking the samples are not high enough to detect any contribution due to the P6 term at temperatures as low as only 4°C above the transition to the cubic state.