A composition, method and kit for quantifying the concentration of one peracid or the total concentration of two or more peracids in a water source are disclosed. In particular, an oxidation sensitive or peroxycarboxylic acid selective leuco dye is used to react only with one or more peroxycarboxylic acids in a water source but not with peroxide usually co-existing in the water source. Not only does the oxidation sensitive or peroxycarboxylic acid selective leuco dye react with peracid(s) but also the reaction generates a color species that can be used visually or through absorbance measurement to quantify the peracid concentration. The method disclosed here is not a titration method, it involves a simple mixing step. The kit or composition disclosed here does not utilize a catalyst, such as iodine.