The structures of two laminar premixed n-heptane/O-2/Ar flames (F1.60: Phi = 1.60, C/O = 0.51, and F1.80: Phi = 1.80, C/O = 0.57) and one laminar premixed n-heptane/methanol/O-2/Ar flame (F1.80 M: Phi = 1.80, C/O = 0.51) are studied at low pressure (4000 Pa) by using synchrotron photoionization and molecular-beam sampling-mass spectrometry (PI-MBMS) techniques. Calculations are performed with a modified chemical mechanism, which satisfactorily simulates the tested flames. The results show that as equivalence ratio increases, the maximum flame temperature is reduced and the flame front is shifted away from the burner surface. The post-flame CO concentration in F1.80 M is lower than that in F1.80, which is attributed not only to the difference in inlet carbon flux but also to the variation in CO formation pathway. As methanol is added, the peak concentrations of C-2-C-7 hydrocarbon intermediates are reduced substantially, and the extent of the reduction in the case of constant C/O ratio is smaller than that in the case of constant equivalence ratio. The production of formaldehyde is promoted with the addition of methanol. Reaction flux analysis indicates that the self-recombination of propargyl radical (C3H3) and the cross reaction between C3H3 and allyl radical (a-C3H5) are the dominant pathways leading from small aliphatics to benzene for all the flames. (C) 2012 Elsevier Ltd. All rights reserved.