Abstract The leaching of toxins from treated wood poses an, as yet, poorly quantified risk to both plant and animal life. In particular, the leaching of pentachlorophenol (PCP) into rainwater falling on treated wood over aquatic environments, such as bridges, is understudied. Computer models have been developed which predict the leaching of creosote from marine pilings. If data were available, similar models could be developed for PCP-treated bridges and the risk to waterways determined for various structural designs. Providing such data is the objective of this study, where the migration of PCP from treated wood under the influence of simulated rainfall was studied using a simulation system that delivered uniform rainfall rates over-treated wood. The runoff from the treated wood was captured and analyzed with high resolution gas chromatography combined with low resolution mass spectrometry. PCP migrated from treated wood into rainwater runoff at a fairly constant rate of approximately 0.15 g l-1 m-2. Small non-significant deviations were observed with rainfall rate, time, and temperature. We suggest that PCP migration rates from exposed treated wood can be modeled and thereby predict the migration of PCP from this source into the environment.