The energies involved in the insulator-conductor transition of polypyrrole in aqueous NO3-, Cl-, Br- and F- solutions, have been directly determined with an isothermal heat flow microcalorimeter. The oxidation was found to be endothermic and potential-dependent, indicating a large heat contribution due to generation of charge carriers in the polymer backbone. Heat changes are sensitive to the presence of additional reactions, such as solvent dissociation and doping with OH- in fluoride solution, leading to overoxidation at high doping levels. Through open-circuit potential decay measurements in F- solution, it was determined that overoxidation involves a one-electron reaction, occurring throughout the polymer.