Pilot scale fermentations with grape pomace from two different wineries were investigated during the 24 weeks of the ensiling period, along with laboratory scale experiments in which the environmental temperatures were held constant at 20, 25, 30 and 35 °C. During this period, yeast and lactic acid bacteria (LAB) counts were made, after which the identity of both groups of organisms was studied, as were the major microbial metabolites present. Major microbial and chemical alterations occurred during the first 3 weeks of ensilage, leaving a more stable product differing significantly from the initial substrate. The results obtained indicated that after initial growth, yeast and LAB populations undergo progressive inactivation at environmental temperatures above 20 °C, although LAB seem to adjust better to this specific, post-fermentation environment. Homogermentative species of Lactobacillus were the dominant LAB. The initial yeast flora of non-Saccharomyces species was replaced by a typical wine yeast flora, i.e. predominantly Saccharomyces cerevisiae. At the chemical level, major alterations were due to an alcoholic fermentation and a malolactic conversion within the first 3 weeks.