The thermal stability in air of 5,10,15,20-tetrakis (4-methoxyphenyl) porphyrin (4-TMPP), 5,10,15,20-tetrakis (3-methoxyphenyl) porphyrin (3-TMPP), and their nickel metallo-complexes (4-TMPP-Ni and 3-TMPP-Ni) has been investigated by thermogravimetry (TG). 4-TMPP and 4-TMPP-Ni exhibit higher thermal stability compared to 3-TMPP and 3-TMPP-Ni. Nickel complexes exhibit a little higher thermal stability than the corresponding porphyrins. The thermal behavior including melting temperature and enthalpy of fusion was determined by differential scanning calorimetry (DSC) and infrared spectra (IR). The activation energies of thermal decompositions of 4-TMPP-Ni and 3-TMPP-Ni were obtained by integral model-free method; the mechanism functions and pre-exponential factors were determined by master plots method. The kinetic models follow the same mechanism function, Avramie-Erofeev equations with integral forms g(alpha)=[-ln (1-alpha)](0.278) and g(alpha)=[-ln (1-alpha)](0.260), respectively. (C) 2005 Elsevier B.V. All rights reserved.