Clay minerals have been identified as reservoirs for polychlorinated dibenzo-p-dioxins (PCDDs) in the environment, and have caused livestock poisoning in several instances when used as feed additives. The congener profiles of PCDDs in clays are dissimilar to those of known anthropogenic (e.g., pesticide manufacture, waste incineration) or natural (e.g., forest fire) sources. Characteristic features of clay-associated PCDDs are the dominance of octachlorodibenzo-p-dioxin (OCDD) as the most abundant congener and very low levels of polychlorinated dibenzofurans (PCDFs). These propensities led to the hypothesis of in situ PCDD formation on clay surfaces. In this study, we examined the formations of PCDDs from several chlorinated organic precursors on clay mineral surfaces. This study attempts to provide the evidence for the clay-catalyzed formation of PCDDs, and a rationale for the predominance of OCDD in natural clay deposits.