The thermochemical and structural properties of penicillin V benzathine tetrahydrate were studied in the temperature interval from 20 to 200-degrees-C. It has been found that, on increasing the temperature, the penicillin undergoes two phase transitions before reaching the melting point. The first is accompanied by complete dehydration and the corresponding structural changes are small, whereas the penicillin’s crystal lattice before and after the second transition, as determined by X-ray diffraction, differs significantly. The overall enthalpy change is 250 J g-1. Both transitions are reversible; the penicillin can be successively heated and cooled an indefinite number of times, retaining its chemical and structural properties, provided that the temperature does not exceed 100-degrees-C. From differential scanning calorimetry data one cannot resolve the kinetic parameters of the two phase transitions; the overall process is second order and the activation energy is 207 kJ mol-1. Carroll-Freeman analysis of the thermogravimetric data indicates that the reaction order of the dehydration is 0.6 and the activation energy is 111 kJ mol-1. All these parameters were obtained at a 1.25-degrees-C min-1 heating rate. The enthalpy of fusion of the anhydrous penicillin is 45 kJ mol-1. The seeming inconsistency of the X-ray data with the DSC data is found to be related to the very pronounced effect of heating rate on the kinetics, and is explained in detail.