This paper proposes a modified pseudoequilibrium calculation, which gives almost the same results as those of kinetic calculations to determine the composition of hydrogen and nitrogen plas,nas at atmospheric pressure. The computing time is tire to three orders of magnitude faster than that of the kinetic calculations. First, according to experimental results, a relationship between the electron temperature T-e and the heavy species one T-h has been proposed. The ratio T-e/T-h varies as a function of the logarithm of the ratio n(e)/n(e)(max), n(e)(max) being the electron density in the plasma core for which equilibrium is achieved (n(e)(max) similar to 10(23)). The kinetic calculations have been performed assuming tire microreversibility where the backward kinetic rate coefficient k(b) is calculated by k(d)/k(b) = K-x, where k(d) is the direct kinetic coefficient and K-x the molar fraction equilibrium constant. When electrons are involved in both direct and backward reactions k(d) and K-X are expressed as functions of T-e. However, when the direct reaction involves electrons while the backward one is due to collisions between heavy species (ol the reverse), a temperature T* between T-e and T-h is introduced T* is determined as a function of the ratio of the electron pur to that of neutral species in such a way that T* = T-e for n(e) > 10(23) and T* = T-h for low values of n(e) (n(e) < 10(15) m(-3)). Compared to hydrogen, the nitrogen composition exhibits a very abrupt variation between 6000 and 6500 K, corresponding to a shift from the dissociation-dominated regime to that of ionization. It occurs because dissociation of nitrogen starts almost simultaneously with its ionization, which is not the case of H-2, for which dissociation is terminated long before ionization starts. If the charge transfer reaction, whose activation energy is low for both gases, is neglected in both cases the electron density increases drastically below 9000 K. These results are quite similar to those obtained when calculating the composition with the multitemperature mass action law. The kinetic calculations are dominated by the reactions with a low activation energy: dissociation, dissociative recombination and charge transfer. Thus, a modified pseudoequilibrium calculation has been introduced, the plasma composition being calculated with the equilibrium constants corresponding to low activation energies [X-2 --> 2X, e + X-2(+) --> 2X, X-2(+) + X --> X+ + X-2 both for hydrogen (X = H) and nitrogen (X = N)] at the temperature T* between T-e and T-h The results are in very good agreement with those of the kinetic calculations.