The mechanism of browning in frozen white and brown A. bisporus stored for 8 months at − 25 °C has been determined based on the level of enzymatic activity, free amino acids, total polyphenols, antioxidant activity and color. The brown variety was less resistant than the white variety to enzymatic browning, due to a higher activity of monophenolase and a minor degree of diphenolase. The mechanism of browning was connected with the regeneration of monophenolase and diphenolase and a reduction in the level of total polyphenols and l-tyrosine. In the white variety, the activity of monophenolase was correlated with the level of l-tyrosine and antioxidant activity, and in the brown variety with the level of l-phenylalanine and parameter a*. Irrespective of the variety, the most effective pre-treatment was vacuum impregnation in sodium metabisulfite and citric acid and then blanching in water. The second pre-treatment in terms of effectiveness was blanching in the sodium erythorbate and citric acid in the brown variety, and blanching in water in the white.