Temperature and species mole fraction profiles have been measured in laminar premixed n-heptane/O-2/Ar and iso-octane/O-2/Ar flames. Both flames have been stabilized on a flat-flame burner at low pressure (6.0 kPa), and species identification and concentration measurement have been performed by mass spectrometric analyses of samples withdrawn locally by molecular beam formation. Temperature profiles were measured by Pt - Pt 10% Rh thermocouples with corrections of the signals to compensate radiative heat losses. A wide range of equivalence ratios extending to 0.7 up to 2.0 has been considered in order to check how the nature of the fuel influences the evolution with this parameter of the species mole fraction profiles. Mole fraction profiles of reactants, major products (CO2, H2O, CO, H-2), main active species (H, O, OH), and small intermediate species (CH3, CH4, C2H2, C2H4, C2H5) have been obtained with working conditions of the MBMS technique usually adopted to study the structure of small fuel molecules. Care was taken to minimize fragmentations in the ionization source of the mass spectrometer and so be able to derive quantitative measurements for intermediate species such as large olefins and alkyl radicals involved in the first steps of the combustion mechanisms of heavy fuels. In this work, C3H6, C4H8, C5H10 and n-C3H7, 1-C4H9, i-C4H9 have been analyzed. The use of very low electron energies in the ionization source of the mass spectrometer limits strongly the accuracy of the mole fraction measurements and aadditional analyses by Gas Chromatography have been performed to control the MBMS data. Emphasis was put as well on the analyses of the species involved in the formation of benzene in hydrocarbon flames: C-3(C3H3, C3H4) and C-4 species (C4H3, C4H4, C4H5) have been analyzed. Comparison of the species mole fraction profiles in flames with identical equivalence ratios points out two main differences: (i) a marked hierarchy is observed in n-heptane flames in the concentrations of the intermediate olefins with ethylene > propene > l-butene, while the corresponding species (ethylene, propene and i-butene) are formed ill very close concentrations in iso-octane flames; (ii) benzene is formed in larger amounts in iso-octane flames. Allene and propyne follow the same evolution, in agreement with the C-3 mechanism now widely accepted to interpret benzene formation in flames.