The structural response of methane/air and methane-nitrogen/air counterflow diffusion flames to strain was investigated by measurements and computations. The numerical predictions were found to be in reasonably good agreement with the experiments. Different reaction pathways leading to PAH formation are examined computationally to obtain a deeper understanding of the process of soot precursor formation in strained diffusion flames. Both experimental and computational results indicate that the concentration of C2H2 and C3H3 as well as that of the PAH, leading candidates for soot precursor formation, diminish with increasing strain rates. The decrease of the PAH is caused by a depletion of the benzene precursors. In looking to find control parameters for strained reactive flows, it is suggested to image strain rates based on the CH2O respectively CHO, to C2H2 ratio. (C) 2001 by The Combustion Institute.