The vascular plant species diversity of the islet groups in the Amvrakikos Gulf and Mesolongi Lagoon, hitherto unexplored, is documented by an inventory of vascular plants, filling gaps in the floristic information available for the two National Parks to which these islet groups belong, as well as for the Ionian land-bridge island system for which detailed studies concerning floristic and phytogeographical relationships were available before. The total vascular flora of the two islet groups comprises 226 plant taxa. Spectra of life-form and chorological elements are presented and discussed. Floristic cross-correlations (beta-diversity) show that the individual islet floras differ from each other from 39 % to 91 %. Concerning the role of eco-geographical variables as influencing plant species diversity in the area, regression analyses indicate island area as the major predictor variable of species richness.