The enthalpies of sublimation of 5,10,15,20-tetraphenylporpbine (CA registry number 917-23-7, TPP) and 5,10,15,20-tetrakis(4-methoxyphenyl)porphine (CA registry number 22112-78-3, TMPP), were determined by the Knudsen effusion method using a quartz-crystal microbalance in a system designed specially to work at high temperatures. The system includes stainless steel-Pyrex glass joints over one of which the effusion cell holder is welded in such a way that the heating and temperature sensor for measurement and control are accessible without venting the system. The design permits precise measurements of the actual sample temperature under low-pressure conditions. From the experimental results the derived values for this property are: Delta(cr)(g)H(m)(o) (TPP, 550 K) = (171 +/- 2) kJ (.) mol(-1) and Delta(cr)(g) H-m(o)(TMPP, 563 K) = (213 +/- 10) kJ (.) mol(-1). Combining these values with the previously reported standard molar enthalpies of formation in the condensed phase, the corresponding standard molar enthalpies of formation in the gas state at T = 298.15 K were calculated as Delta(f)H(m)(o)(g) = (839 +/- 9) kJ (.) mol(-1) for TPP and Delta(f)H(m)(o) (g) = (295 +/- 14) kJ (.) mol(-1) for TMPP. An estimate of these enthalpies by the second-order Benson's group additivity approach was computed based upon the assumption of total or null aromaticity in the porphyrin macrocycle. Using the enthalpies of formation of the gaseous atoms in their ground states, the enthalpies of atomisation were derived as: DeltaatH(m)(o)(g) = (39125 +/- 22) kJ (.) mol(-1) for TPP and Delta(at)H(m)(o)(g) = (45277 +/- 25) kJ (.) mol(-1) for TMPP. Recommended Laidler parameters were used to predict these values, considering a correction for four pyrrole ring strain contribution.