The temperature dependence of electron transfer quenching of triplet excited neutral Pd-tetraphenylporphyrin is investigated in a series of isotropic solvents and in a nematic crystalline one. Aromatic nitro compounds and quinones of different redox potentials are used as quenchers so that the quenching reaction is exergonic to slightly endergonic. The reactions of quenchers with the largest negative Delta G(0) are diffusion controlled and thus have a positive enthalpy of activation, the quenchers with moderately negative Delta G(0) values quench in the activation controlled realm with negative activation enthalpies, and the weakest quenchers again show positive activation enthalpies. An isokinetic relationship is found only in isotropic solvents, the first case of the isokinetic temperature being well above the temperatures of the experiments. Reactions with negative activation enthalpy are generally considered to have a pre-equilibrium, but it is shown that an elementary reaction may exhibit a negative enthalpy of activation, too, if the reaction entropy is negative enough. It is conjectured that the negative enthalpies of activation observed in this work are due to an elementary reaction.