An isocratic HPLC method routinely used in the National Laboratory for Food and Drug Analysis of Taiwan was validated for the simultaneous determination of six aminophenols and phenylenediamines in commercial hair dyes. After extraction of the commercial hair dye product, the dye intermediates were determined by HPLC. Recoveries from the extraction were between 91.6 and 96.5%. The method was then evaluated in an interlaboratory collaborative study according to AOAC guidelines. Five laboratories in Taiwan participated in the study that analyzed the test product, which was preanalyzed by two laboratories to ensure acceptable homogeneity. The RSD(r) and RSD(R) values of the measurements obtained for the dye intermediates in the product were < or = 3.75 and < or = 5.95%, respectively. The method demonstrated acceptable reproducibility, as evidenced by HorRat values of 0.82- 0.97. The applicability of the method to the determination of oxidative hair dye components was further demonstrated in analyses of two different products. The method is thus proposed to be used by manufacturers and laboratories to evaluate the quality of commercial hair dyes containing the six aminophenols and phenylenediamines.