Therefore, the aim of the present work was to evaluate the use of aqueous extract of common onion (Allium cepa L., yellow variety) to stabilize colour in frozen Agaricus bisporus (Lange) Sing. mushrooms. The study compared the effectiveness of vacuum soaking in aqueous extract of onion with vacuum soaking in a solution containing sodium metabisulphite (widely used in the processing industry), and a solution containing citric and L-ascorbic acids.