The formation mechanism of polychlorinated dibenzo-p-dioxins (PCDDs) has been studied for the first time by ab initio calculations. We examined twelve possible formation pathways from 2,4,5-TCP to three PCDD isomers (2,3,7,8-TCDD, 1,2,4,7,8-PeCDD, and 1,2,4,6,7,9-HxCDD). Four pathways by direct intermolecular condensation of two 2,4,5-TCP molecules and eight pathways via radicals were considered. We found that 2,3,7,8-TCDD is the only one of the three PCDD isomers produced when the reactions proceed by direct condensation from 2,4,5-TCP. However, when the reactions proceed through radicals, two path types seem to compete with each other. At low temperatures, one path type prefers to form TCDD, and the other prefers to form PeCDD.