Current toxicogenomic approaches generate transcriptional profiles that can be used to develop gene expression signatures of environmental toxicants and address their mechanisms of action. It has become evident however, that the intricate processes governing gene transcription are overlaid with a complex set of molecular instructions involving epigenetic modifications. These commands regulate both gene expression and chromatin organization through coordinated sets of histone modifications and heritable DNA methylation patterns. While the effects of specific environmental toxicants on gene expression are the subject of much study, the epigenetic effects of such compounds are poorly understood. Here we have used human promoter tiling arrays in conjunction with chromatin immunoprecipitation to identify changes in histone acetylation profiles due to chemical exposure. Chromatin from cells exposed to an industrial pollutant, the polyaromatic hydrocarbon benzo(a)pyrene, was immunoprecipitated with antibodies against acetylated histones. Affymetrix promoter tiling microarrays were probed to generate epigenomic profiles of hypo- and hyperacetylated chromatin localized to gene promoter regions. Statistical analyses, data mining and expression studies revealed that treated cells possessed differentially acetylated gene promoter regions and gene-specific expression changes. This ChIP-on-chip approach permits genome-wide profiling of histone acetylation patterns due to chemical exposures that can be used to identify chromatin-related signatures of environmental toxicants and potentially determine the molecular pathways these changes target. This approach also has potential applications for profiling histone modifications and DNA methylation changes during embryonic development, in cancer biology and in the development and assessment of cancer therapeutics.