This study investigates the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) in oxidation of 4,4′-dichlorobiphenyl (4,4′-DCB) under gas phase conditions over a temperature range of 400-800°C. The experiments involved an isothermal flow reactor equipped with a sample collection system to adsorb PCDD/F. We coated the walls of the reactor, made of high purity quartz, with boric oxide to remove catalytic effects and employed the gas chromatography-ion trap mass spectrometry (GC-ITMS) to quantitate PCDD/F. Trace analysis by GC-ITMS identified all four MCDF and up to five DCDF isomers, of which 3,7-DCDF constitutes the dominant species, formed directly from 4,4′-DCB. MCDD and DCDD appeared at lower concentration, generated from condensation of chlorophenoxy and chlorobenzenes. Our results show significant differences from the measurements of the thermal decomposition of polychlorinated biphenyls (PCB) performed in sealed ampoules, indicating strong surface effects present in earlier studies. Gas-phase reactions (> 600°C) lead to selective formation of a small number of lower-chlorinated pollutants, whereas reactive surfaces, as evident from literature, engender catalytic (but non-selective) appearance of a large number of higher-chlorinated and toxic chloroaryl congeners, even at temperature of less than 300°C. Results from the density functional theory (DFT) calculations indicate that, majority of the detected congeners of PCDF arise from the oxidative transformation of the parent 4,4′-DCB; i.e., not from the commonly suggested precursors of chlorinated phenols and benzenes. Our findings have significant importance to understanding the emission of PCB and PCDD/F in combustion processes.