The hydrodemetallation (HDM) reactions of nickel-5,10,15,20-tetraphenylporphyrin (Ni-TPP) and vanadyl-5,10,15,20-tetraphenylporphyrin (VO-TPP) were studied in the absence of a catalyst. Ni-TPP as well as VO-TPP is found to demetallate through a reversible sequential reaction mechanism in which similar hydrogenated intermediate species are formed, As compared to catalized HDM, reaction rates are low. Hydrogenation reactions of Ni-TPP are first order with respect to the liquid phase concentration of hydrogen. Hydrogen sulfide promotes the conversion of Ni-TPP. An equilibrium constant for the reversible hydrogenation of Ni-TPP to nickel-5,10,15,20-tetraphenylchlorin (Ni-TPC) has been estimated as a function of temperature. A heat of reaction of -97 kJ/mol was found for the reversible hydrogenation of Ni-TPP to Ni-TPC, Comparison of the reactivities of a metal-free porphyrin (H-2-TPP), Ni-TPP and VO-TPP revealed that the metals have a stabilizing influence towards hydrogenation and ring fragmentation of porphyrin macrocycles.