Summary: Late-stage acid pegmatites veining the predominantly ultrabasic Currywongaun-Doughruagh intrusion are frequently associated with amphibole-rich reaction borders in the adjacent wall-rocks. The chemical and mineralogical changes resulting from reaction between an acid pegmatite vein and the orthopyroxenite wall-rock are assessed from chemical analyses of rocks and microprobe analyses of the amphiboles forming the zoned reaction border. The main chemical change was the addition of H2O to the wall-rock; otherwise the formation of the anthophyllite and hornblende of the reaction border from pyroxene was virtually isochemical. Enrichment in other chemical species, notably Ca and AI, was restricted to the region close to the interface between the reactants. This inhibited style of reaction is attributed to the initial marginal crystallization of the acid pegmatite melt acting as a barrier to subsequent metasomatic interchange.