The micro-structure of a premixed, atmospheric-pressure, fuel-rich (equivalence ratio 2.6 ), flat flame of methane has been determined experimentally using a heated micro-probe coupled to on-line gas chromatography/mass spectrometry (GC/MS ). The identities and absolute concentrations of over 40 major and minor species have been established, including a large number of aromatics, substituted aromatics and polycyclic aromatic hydrocarbons (PAH ) by the direct analysis of samples withdrawn from the flame without pre-concentration. Mole fractions of the species quantified were in the range 0.45—2.0 × 10−9. The largest PAHs detected were the family of C20H12s (molecular weight 252 ), that include benzo (a )pyrene, perylene and benz (e )-acephenanthrylene, with peak concentrations reaching 1 ppmv. The results show the preferential formation of fused PAHs with even number of aromatic rings over PAHs with odd number of rings. The PAHs detected also suggest that they can be formed by at least 3 parallel routes; synthesis via the sequential addition of small building blocks, such as C2H2, by the recombination of smaller PAHs, or via isomerization. The measurements provide information on the identities and levels of hazardous air pollutants formed in combustion processes and represent useful new data for the development and validation of detailed reaction mechanisms describing their origin and fate.