Conventional approaches for monitoring the effectiveness of wastewater treatment processes include evaluating the degradation of the target compound and/or generation of its nontoxic byproducts. These approaches are, however, limited because routine chemical analyses alone are neither able to fully address potential hazard to biological receptors nor characterize potential synergistic interactions. This study was carried out to investigate the degradation effectiveness of pentachlorophenol (PCP) by treatment with UV-A, UV-B photolysis, sunlight, TiO(2) photocatalysis, and/or their combinations. Chemical analyses of the parent compound and its selected byproducts, as well as acute toxicity assessment using the luminescent bacteria Vibrio fischeri (Microtox), were conducted during and after the various photolytic and photocatalytic treatments. In general, the toxicity reduction pattern observed after treatment corresponded well with the chemical degradation data. However, it should be noted that there were occasions that acute microbial toxicity was observed even from the treated water samples, some of which showed complete removal of the parent compound. This post-treatment toxicity might be due to toxic PCP byproducts, which may include polychlorinated dibenzodioxins/furans, tetrachloro-p-benzoquinone, and other intermediates. The TiO(2) photocatalysis with UV-B photolysis was the most effective method to remove both PCP and its toxic derivatives in the water. The Microtox assay is an easy to use and promising approach for evaluating the effectiveness of wastewater treatment processes.