Aim: Epiphytic plants are isolated from each other by nonhabitat canopy elements and are thus expected to act as islands, the biodiversity of their inhabitants (e.g., spiders) conforming to island biogeographic predictions of species-richness patterns. Although it has been shown that arthropod diversity decreases with decreasing epiphyte size, the effects of isolation have not yet been addressed. We studied the joint effect of isolation, spatial position, and size of epiphytic plants (canopy islands) on species richness, relative rareness, and similarity of spider communities. Location: A shade-coffee plantation in central Veracruz, Mexico. Taxon: Spiders (Araneae), vascular epiphytes (Bromeliaceae, Piperaceae, Orchidaceae, Araceae, Pteridophyta). Methods: We collected all canopy islands occurring on three trees and recorded their three-dimensional spatial position. In the laboratory, we disassembled the plants and collected all spiders present. We analysed the effects of island size, isolation, and spatial position on the species richness and the relative rareness of spider communities using generalized linear models (GLM), on the distribution of different spider guilds using a CCA, and on community composition using a permutational multivariate analysis of variance (Permanova). Isolation and spatial position were addressed using five distance measures representing isolation from different potential species sources. We tested how the similarity in spider community composition changed with the distance between islands using Mantel correlograms. Results: As predicted by island biogeography theory, spider species richness increased with canopy-island size and decreased with isolation. In comparison to island size, the effect of isolation was weak, though significant. Relative rareness was hardly affected by island size but more by isolation. Compositional similarity was affected by island size and decreased with increasing spatial distance up to ca. 4 m. Guilds separated along the main CCA axes, this ordination being driven by epiphyte size and position. Main conclusions: Epiphytic plants behaved like canopy islands in that their size and isolation influenced the diversity and composition of spider communities. However, the effect of isolation was only a fraction of that of island size, perhaps because spatial relationships are taxon-specific. This may be due to differences in hunting behaviour and dispersal capacities, for example, between guilds of hunting and web-building spiders, which is a dimension deserving more attention. For a better understanding of biogeographic principles driving the diversity of canopy island inhabitants, further research on this topic should include position and isolation, at scales matching the mobility of different functional groups, as part of their explanatory variables.