The perturbation of a flat, premixed, fuel-rich propene/oxygen/argon flame at 50 mbar by a sampling nozzle used in typical molecular beam mass spectrometry (MBMS) experiments was analyzed. Two-dimensional nozzle effects were visualized with laser-induced fluorescence (LIF) of OH, and the influence of the nozzle on the temperature profile along the burner centerline was examined using LIF of seeded NO, both in the flame with and without the nozzle. Temperature deviations of the order of 500 K were observed. For the nozzle-perturbed flame, two strategies for the determination of temperature effects were pursued. First, temperatures were only measured directly in front of the nozzle for different nozzle positions, as in previous work. Secondly, individual temperature profiles at different heights above the burner were determined for each nozzle position, thus simulating the conditions for a nonreacting species sampled by the nozzle along the burner centerline. To investigate the influence of the nozzle on the flame structure, the latter was modeled using the measured flame temperature profiles, with and without the nozzle. Peak concentrations and the positions where they are a maximum are compared for a variety of species, and concentration profiles are analyzed for selected examples, in association with the experimental results. The consequences for a meaningful comparison of MBMS data with flame model calculations are discussed with particular attention to the formation of C-6 species (benzene) in this fuel-rich propene flame. (C) 2000 by The Combustion Institute.