Micrometer-sized precursor soot particles were formed by pyrolysis of pure atmospheric-pressure acetylene in a flow tube reactor. Real-time analysis of both the gas and particle phases of the reactor effluent was performed with an ion trap-based aerosol mass spectrometer. Off-line analyses were also performed on the filter-collected reactor effluent. The real-time particle mass spectra revealed the presence of a polymeric mass distribution starting near 1000 Da and continuing to greater than 1750 Da. The spacing between the polymer peaks is approximately 70 Da. Comparison of real-time and off-line analyses reveals the metastable nature of the polymer-like high mass species. (C) 2004 Elsevier B.V. All rights reserved.