Roméite (Ca, Fe, Mn, Na)2(Sb5+, Ti4+)2(O, OH, F)7 is a rare mineral found in metamorphic iron-manganese deposits and in hydrothermal Sb-bearing veins. It is isostructural with the pyrochlore-group minerals of the general formula A2-mB2X6-wY1-n · pH2O. The pyrochlore-group minerals are important Nb and Ta ores, and are also used as an actinide host phase in radioactive waste. The crystal chemistry of roméite from the type locality Praborna (Italy), from Massiac (France), and from four newly discovered localities in the Swiss Alps, and of "lewisite", a questionable species related to roméite from Tripuhy (Brazil), is compared to that of pyrochlore. A wide range of substitutions has been observed including (1) independent substitutions on the A- and B-sites, and (2) coupled substitutions between the A- and B- and between the A- and Y- sites. Only the roméite from Massiac, derived from weathering of stibnite, contains significant H2O (up to 14 wt%). The A-site vacancies in roméite appear to be controlled by the primary conditions of crystallization, and not by post-crystallization alteration. The Y-site chemistry of roméite varies from locality to locality; it can be dominated by F, OH, or be fully vacant. The "lewisite" octahedral crystals studied are a sub-microscopic mixture of roméite with a mineral structurally related to pyrochlore, which grows at the expense of roméite.