The objective of the study was to evaluate the ability of methods to quantitatively measure a low concentration of a mixture of phenols. The study consisted of four concentrated solutions shipped in sealed glass ampoules to various laboratories. The participants were instructed on receipt to dilute 5 ml of each sample to 1 liter with a good quality (phenol free) distilled water, and were informed that the samples would then contain concentrations of phenol and 2,4-dichlorophenol in the range of 1.0 to 200 microgram per liter. The distillation and chloroform extraction (of the final color) method will measure the phenol content of a sample very accurately and precisely, but it is not as sensitive as is desired. On the other hand the simplified aminoantipyrine method will measure neither phenol nor 2,4-dichlorophenol very accurately or precisely.