The paper provides a comparative analysis of three methods used to determine D_m diffusivity, which were tested for non-stationary desorption process and referred to various stages of its course. These are: the √t-type method – linked to the initial phase; the logarithmic procedure – for the advanced phase of the process, and the half-time method – referring to the point of time when 50% of mass has been exchanges. The article presents results of desorption process research carried out in the temperature of 20°C and encompassed three mortars characterized by the following water-cement ratios w/c = 0.50, 0.65 and 0.80. Samples of these mortars were conditioned for several months under air relative humidity φ= 97% until the time of reaching moisture equilibrium, and then they were placed in humidity conditions of φ≈ 0%. The values Dm(√t), D_m(ln), D_m(t_0.5), obtained through the analyzed desorption process (T = 20°C, φ= 97% → 0%), presented a good conformity of all the three tested methods in the case of all the mortars – from the fully identical results: D_m(t_0,5)/D_m(ln) = 1.00 with w/c = 0.80 to the most diversied ones: D_m(t_0,5)/D_m(√t) = 1.23 with w/c = 0.65.

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