A modification of the CHEMKIN II package has been proposed for modeling addition of an arbitrary species at an arbitrary temperature to an arbitrary distance from the burner along a flat flame. The modified program was applied to the problem of addition of acetylene or benzene to different positions of a 40-Torr, phi = 2.4 benzene/O-2/40%-N-2 premixed flame to reach final equivalence ratios of phi = 2.5 and 2.681. The results obtained showed that acetylene addition to early positions of the flame led to significant increase in pyrene production rates, but pyrene concentrations were lower in the flames with acetylene addition in both the phi = 2.5 and 2.681 cases. Addition of benzene to the flame did not alter pyrene production rates in either the phi = 2.5 or 2.681 cases; however, for phi = 2.5, pyrene concentrations increased with benzene addition, while for phi = 2.681, pyrene contents decreased in comparison to the correspondent flames with no addition. Acetylene addition led to a significant increase in pyrene production rates, but the pyrene levels dropped due to increase in the flow velocity. Pyrene production rates were not sensitive to benzene addition, but pyrene contents increased with benzene addition when the flow velocity decreased. These results show that PAH concentration changes accompanying species addition to flames should be interpreted carefully, because an increase or decrease in the content of a PAH species does not necessarily reflect an effect on its formation rate or mechanism. (c) 2006 The Combustion Institute. published by Elsevier Inc. All rights reserved.