The bacteria of the intestine have to cope with varying osmotic conditions in their ecosystem. In this in vitro study, the modified Hohenheim gas test (HGT) was used to determine fermentation activity and bacterial composition of pig's faecal microbial inoculum, when fermenting different carbohydrates (inulin, corn starch, potato starch, cellulose, pectin), under normal buffered and osmotic stress conditions (elevated medium salinity). After 24 h of fermentation, gas, ammonia and short-chain fatty acid (SCFA) production was measured, and the cell numbers of total eubacteria, Lactobacillus spp., Bifidobacterium spp. and enterobacteria were analysed, using real-time polymerase chain reaction. There was a significant reduction in gas production after 24 h when comparing osmotic stress conditions with normal buffered conditions, and there were also differences among carbohydrates under both conditions. The content of SCFA was significantly lower when comparing osmotic stress with normal buffered conditions. Under osmotic stress, inulin and corn starch increased (p < 0.05) cell numbers of total eubacteria, while Bifidobacterium spp. and enterobacteria were higher (p < 0.05) when corn starch and pectin were fermented, respectively, in comparison to the other carbohydrates. The in vitro system of the modified HGT appears to be suitable to scrutinise effects of carbohydrates on the metabolic activity and composition of the microbial community under osmotic stress conditions, as they might occur during situations of osmotic diarrhoea.