Gradients of humidity on tropical high mountains are reflected by shifts of vegetation belts and changes in species composition and vegetation structure on the wind- and leeward sides. The superpáramo flora and vegetation were studied on opposite sides (west and east) of two Ecuadorian mountains, Chimborazo and Antisana. Zonal vegetation was studied in vegetation samples set out in a semirandom way, and the data were complemented by species occurring in azonal habitats. Ordination and clustering techniques were used to analyze the vegetation samples, while two different measures of the floristic similarity (index of quantitative floristic similarity and comparison of the observed and expected numbers of species shared between the mountain pairs) were employed to evaluate the floristic relationships between the study sites. The superpáramo belt occurs asymmetrically, with regard to altitude, on the two mountains, being generally lower on the eastern side. Slopes with corresponding aspects between the mountains were more similar than were the opposite slopes of each mountain. The observed floristic pattern is interpreted in the context of the precipitation gradient leading to a rain-shadow on the western side of the mountains. The occurrence of the desert-like area, the so-called Arenal Grande, and the unique páramo vegetation on the western side of Chimborazo is discussed.