Terminal ring isomers of the reactive anthraquinone dye C.I. Reactive Blue 2 were synthesised and the reaction product and potential hydrolytic by-products analysed by thin-layer chromatography and reversed-phase high-performance liquid chromatography using N-cetyltrimethylammonium bromide as an ion-pair reagent. These techniques provided excellent resolution of potential chromophoric contaminants and showed the predominant formation of a single dye product in each case. The crude dyes were purified to a high degree (>99%) by chromatography on Sephadex LH-20, providing high-quality dye preparations for use in protein binding studies. A number of commercial dye preparations were also analysed and found to be composed primarily of the ortho-sulphoanilino isomer. © 1988.