Peroxiredoxins are cysteine-dependent peroxidases that react with hydrogen peroxide, larger hydroperoxide substrates, and peroxynitrite. Protocols are provided to measure Prx activity with peroxide by (1) a coupled reaction with NADPH, thioredoxin reductase, and thioredoxin, (2) the direct monitoring of thioredoxin oxidation, (3) competition with horseradish peroxidase, and (4) peroxide consumption using the FOX assay.