We investigated the importance of water chemistry and water regime for vascular plant and bryophyte species distribution in Western Carpathian mires dominated by Sphagnum. Seventy-seven small circle plots distributed across a wide geographical area, a wide range of mineral richness and all possible microtopographical features were sampled in terms of species composition, physical-chemical water properties and water regime during one growing season. Both water chemistry and water regime were found to be important factors for vegetation composition. Bryophytes reflected only one clear gradient, connected to base- richness (pH, conductivity) and maximal water-level, whereas three different environmental gradients determined the occurrence of vascular plants: water-level amplitude, base-richness and an indistinct gradient presumably connected to peat layer thickness. When the entire data set was subjected to DCA ordination, the first resulting axis was governed by the bryophyte subset, whereas the second one was governed by the vascular plant subset. The species density of vascular plants was positively correlated with pH and conductivity. On the contrary, bryophyte species density showed no relationship to environmental factors. We further compared the pH values measured in groundwater and in water squeezed from bryophytes from the same plot; these plots were distributed along the base-richness gradient. Only in the acidic mires did the use of squeezed-water chemistry in the analyses give results similar to the use of groundwater pH. Further, we found that Sphagnum species with a similar response to the base-richness gradient had differentiated niches with respect to the water level gradient and vice versa. Sphagnum contortum and S. warnstorfii exhibiting the same demands for groundwater pH were segregated along the gradient of maximum water level. An analogous pattern was detected for acidophilous species Sphagnum magellanicum and S. papillosum.