The article presents results on the diversity of vegetation in flooded roadside areas in Karelia on mineral and peat soils. About 200 roadside sites were surveyed, and 297 relevé descriptions of mire and hygrophilous herbaceous vegetation were produced. The vegetation diversity in the studied sites includes 17 associations, 13 of which belong to the herb, 3 to the herb-Sphagnum, and 1 to the shrub-herb vegetation classes. The environmental factors (moisture, light intensity, soil pH, and soil mineral nitrogen) driving vegetation differentiation in these associations were identified using gradient analysis and Ellenberg's indicator values. Nonmetric scaling was applied to plot the ordination diagram for the distribution of the associations. A correlation was established between the ordination axes and environmental factors. The first axis is the principal gradient, explaining 42.5 % of the total variance, while the second axis explains 33.9 %. The first axis correlates with soil mineral nitrogen content (r = –0.795) and soil pH (–0.787). The second ordination axis correlates with moisture (–0.585) and illumination (–0.509). In the ordination plot and cluster analysis output, the syntaxa form four clusters corresponding to the degree of soil mineralization, ranging from eutrophic herbaceous communities on mineral clay soils (Scirpetum sylvatici, Phragmitetum australis, Typhetum latifoliae, Caricetum acutae, Filipenduletum ulmariae, Equisetetum fluviatilis) to mesotrophic herb-Sphagnum communities on peat deposits (Comareto – Sphagnetum, Equiseto fluviatilis – Sphagnetum, Cariceto rostratae – Sphagnetum).