The objective of the research study was to determine the effect of soy whey on fermentation properties of wheat flour of type 750 and rye flour of type 720. Soy whey was added as a substitute of water and its amount was 50 % and 100 % by weight of water used in the control sample prepared without soy whey. I was found that the soy whey added had a favourable effect on the fermentation properties of flour. In particular, positive results were obtained for the 1: 1 mixture of soy whey and water. In the latter case, it was found that the total quantity of carbon dioxide released during fermentation of wheat flour dough increased by 9 % and that of rye flour dough by 10 %, and the quantity of gas trapped in the doughs analysed increased by 4 and 9 %, respectively. In the case of replacing 100 % of water with soy whey, it was possible to significantly reduce the fermentation time of wheat flour to the optimum growth of 38 % and that of the rye flour dough to 11 %. No apparent changes were found in the quality of the wheat and rye gruels with the soy whey added. Only a slight increase in the initial gelatinization temperature was reported for all the suspensions made with the soy whey added. A 50 % solution of whey added to the flour caused the amylolytic activity in the sample to increase, and the result thereof was a reduction in the falling number of wheat flour by 3 % on average, and that of rye flour by ca. 13 %. The research study performed suggests that the soy whey can be used as a natural regulator of the gas-generating ability of flours, especially of rye flour.