Results of comparative analysis of four sets of experimental data, cited in the literature equa-tions, are presented. Two packages of statistic software – SAS and STATISTICA were used for this purpose. It was stated that the differences in computed equation coefficients and errors in the approximation occurred. With the application of GAB, Halsey and Oswin equations, the differences were insignificant, whereas Chung-Pfost and especially Henderson equations gave quite essential differences, amounting to 8% of absolute error and 36% of relative error, for equilibrium humidities of the wet material. The differences were increasing with the decreasing temperature. The application of the iterration step, smaller than nominal, in STATISTICA software resulted in better coincidence of approximation results for the both programs. The differences found in this work suggest the necessity of development of unified methodology for calculation of measurement results.