The goal of this work is to explore soot particle composition as it develops in flames. The paper presents three main findings: (a) A comprehensive soot particle mass spectrum up to similar to 1000 m/z, including positive and negative fragment ions, (b) the impact of particle size on mass spectral lineshapes, and (c) an examination of soot composition as a function of fuel, height in flame, equivalence ratio, and fuel/air flow rate. The positive ion spectrum reveals a long sequence of C(x)H(y)(+) fragments, a wide range of stabilomer PAHs extending from pyrene to beyond coronene, and an extensive series of fullerenes from C(32)(+) to C(80)(+). The negative ion mass spectrum includes simply a weak series of C(x)(-) and C(2x)H(-) ions. Positive and negative ion peaks are found to exhibit distinct particle size dependent lineshapes characteristic of their particulate origins. The overall ion count tracks soot volume fraction with height above the burner and fuel/air ratio. But the composition revealed by the fragment ion pattern is relatively independent of these parameters and to the choice of ethylene, acetylene, ethane, and methane fuels. These observations are discussed in the context of the mechanisms for soot growth. (C) 2009 Elsevier Ltd. All rights reserved.