The applicability of molecular beam mass spectrometry (MBMS) in studying the structure of counterflow flames has been tested by investigating a counterflow flame of CH4/N-2 and O-2/N-2. The thermal structure of the flame was examined using a microthermocouple; the concentration profiles of such stable species as CH4, O-2, and CO2 were measured by sampling with a microprobe and MBMS at various positions. The microprobe did not disturb the flame. However, the sonic probe, when inserted into the flame transverse to the burner axis to measure the centerline concentration profiles, produced a significant disturbance of the flame. But no such disturbance was observed when the tip of the sonic probe was located at the periphery of the burner. Good agreement was obtained between the concentration profiles of stable species, as measured using a microprobe and a sonic probe at the periphery of the burner. To verify the applicability of MBMS for detecting radicals and other labile species in a counterflow flame, the concentration profiles of H, OH, and the main phosphorus-bearing species in the counterflow flame doped with trimethylphosphate (TMP) were measured by MBMS at the periphery of the burner and compared with results of modeling using the OPPDIF code and a mechanism for the combustion of TMP, tested previously in premixed flames of methane and oxygen with TMP as an additive. Good agreement was obtained between the measured and simulated concentration profiles for the reagents, as well as for the final and intermediate products with relatively high molecular weights (PO, PO2, HOPO, HOPO2). The measured concentration profiles of species with low molecular weights (H2O, CO2, OH, H) were found to be broader than the calculated ones-in fact, the lower the molecular weight, the wider was the profile. This is probably due to a real flame not being one-dimensional. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.