The rate constant of the four-site cyclic chemical exchange between pyrrole protons in tetramesitylporphyrinatoiron(III)bis(2-methylimidazole) has been measured in the temperature range 236-255 K from H-1 transverse relaxation times (T-2) observed at the 500 MHz field strength. The values of the rate constant were obtained through the simulation of the observed T-2＇s under the assumption that their values are dominated by the rate of the chemical exchange. The measurement of the exchange rate constant was also attempted at a lower temperature (230 K), but the use of extrapolated intrinsic T-2＇s at that temperature introduces a significant error into the simulated rate constant. The thermodynamic activation parameters of the cyclic exchange were determined as Delta H double dagger = 48 +/- 1 kJ/mol and Delta S double dagger = -10 +/- 6 J/K . mol. The value of k varied from 30 to 270 s(-1) in the temperature range 236-255 K. The near-zero exchange activation entropy is consistent with two previous studies but contradicts another study based on saturation-transfer measurements previously carried out by the authors of this work. A comparison of all available studies shows that the saturation-transfer experiments overestimate the value of the exchange rate constant by up to a factor of 2, It is shown that equilibrium with the high-spin form of the porphyrin complex does not affect the observed T-2＇s in the temperature range used in this study but may do so at temperatures above 270 K. It is also shown that neither the linearity of the Curie plot nor the behavior of the T-1 versus temperature plot should be considered indicators of the absence of the low-spin reversible arrow high-spin equilibrium.