In the Neotropics, differences in phytogeographic patterns and species richness respond non-randomly to edaphic factors in many upland forests. However, whether wetland forests follow the same species differentiation patterns is not well known. In this paper we analyze the relationship among species occurrences and selected soil characteristics, particularly salinity and acidity, in several wetland plant communities. Plant specimens were collected and soil samples analyzed for exchangeable cations, chloride, and organic carbon. Statistical methods were applied to assess β-diversity and to detect relationships between patterns of floristic variation and spatial variations of the soil conditions. β-diversity was high among vegetation communities. Edaphic conditions were heterogeneous, and landward gradients were present only for salinity and some exchangeable bases. This resulted in a lack of a straightforward relationship of vegetation patterns to salinity. More than 30% of the reported species covaried with at least one of the three acidity-related variables, suggesting that some tolerance to deleterious effects of H and Al³ at pH values < 4 is likely. Thus, in contrast to what we might expect in deltaic regions, acidity rather than salinity was the major factor driving species organization. However, variation of these soil characteristics taken together accounted for only 40% of floristic differences among communities.