Measurements of X-ray diffraction, electrical resistivity, and magnetization are reported across the Jahn-Teller phase transition in LaMnO 3. Using a thermodynamic equation, we obtained the pressure derivative of the critical temperature (TJT), dTJT/dP = -28.3 K GPa-1. This approach also reveals that 5.7(3)J (mol K) -1 comes from the volume change and 0.8(2) J (mol K)-1 from the magnetic exchange interaction change across the phase transition. Around TJT, a robust increase in the electrical conductivity takes place and the electronic entropy change, which is assumed to be negligible for the majority of electronic systems, was found to be 1.8(3) J (mol K)-1. © 2011 Taylor & Francis.