We studied polycyclic aromatic hydrocarbon (PAH) formation in laminar counterflow CH4-N-2/O-2-N-2 and C3Hg-N-2/O-2-N-2 nonpremixed flames using a gas chromatograph-mass spectrometer (GC-MS) under widely varying experimental conditions. In this paper we describe the effect of varying the flame-controlling parameter on the formation of PAHs. The formation of most PAHs is enhanced by increasing the strain rate regardless of fuel species and the stoichiometric mixture fraction, Z(st). Increasing the dilution levels slightly suppresses PAH formation. However, the effect is quantitatively weak. PAH formation also is suppressed as Z(st) increases. However, the quantitative dependence of PAH formation on Z(st) variation is different between CH4 flames and C3H8 flames. Naphthalene is the most abundant compound under all experimental conditions in CH4 flames. On the other hand, for C3H8 flames, naphthalene is the most abundant compound in the range of smaller Z(st) while acenaphthylene is abundant under larger Z(st) conditions. For all experiments, under small Z(st) conditions, small-molecular-mass PAHs including naphthalene have significant concentration. Large-molecular-mass PAHs including acenaphthylene, which are-mostly 2- and 3-ring, are produced mainly for larger values of Z(st), when nonpremixed flames are established in the fuel stream. The formation behavior of PAHs in the counterflow nonpremixed flame is classified into two groups for C3H8 flames. One is the PAH group that has formation behavior similar to naphthalene, and the other is the PAH group that has formation behavior similar to acenaphthylene. It is experimentally observed that the effect of varying the initial C/O ratio on the PAH formation of nonpremixed flames is similar to that of previously reported numerical predictions for C2H2 premixed flames. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.