Reactive dyes, and their hydrolysis and reaction products, can be determined at the nanomolar level using cathodic stripping voltammetry (CSV) with adsorptive accumulation. The potential of the technique in the monitoring of reactions of reactive dyes with textile fibres and a range of chemical compounds, in addition to its use in monitoring reactive and spent dyes in environmental samples, is discussed. The CSV of Procion Blue MX-R and Cibacron Blue 36A is used as illustration: significant changes in the voltammograms on hydrolysis or methanolysis allow the reactions to be monitored.