The paper presents the physicochemical composition, rheological properties and results of fermentation of high-gravity corn mashes (extract content of about 20, 23 and 25% w/w) prepared by pressureless liberation of starch (PLS). Fermentation was conducted for 3 days at 28-30oC, by using dried distillery yeast strain As4 (Saccharomyces cerevisiae). Periodically, samples of mashes were collected in order to monitor fermentation parameters (measurement of apparent and true extract, ethanol concentration, loss of sugars). The obtained results showed a significant effect of corn mashes extract on their rheological properties and physicochemical composition, both of sweet mashes (extract, reducing sugars, dextrins, viscosity) and mashes after fermentation (apparent extract, true extract, ethanol content, reducing sugars, dextrins, viscosity). Fermentation of corn mash with the initial extract of about 20% w/w in the 3-daily system resulted in its apparent extract on the level of 0% w/w. Increasing the mashes density to 23 and 25% w/w resulted in an increase of their viscosity and concentration of sugar substrates. Their apparent extract after completion of the process amounted to 2.4 ± 0.1% w/w and 4.4 ± 0.1% w/w, respectively. Moreover, lower yields of fermentation process (by 16% and 21.4%) and lower degree of sugar assimilation (by 6.9% and 9.3%) compared to the sample with the extract of 20% w/w were observed.