Polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs) are undesired byproducts that form as microcontaminants during the synthesis of pentachlorophenol. Adding small amounts (at least 100 ppm) of Ni powder to the reactor reduces the toxic equivalent (TEQ)) concentration in the product to about 70% of what it would be without Ni. By adding Ni powder at the start of the run and terminating the chlorine flow near the end of the run, pentachlorophenol with a TEQ concentration of about 0.4 ppm can be repeatedly synthesized in the laboratory. Co and Mn are about as effective as Ni in inhibiting the formation of toxic microcontaminants, but Mo, W, Ti, Cu, Fe, and Zn have no measurable effect on the TEQ concentration.