The chain-end-group composition was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analyses of low-molecular-weight polystyrene (PS) and poly(methyl methacrylate) (PMMA) produced by free-radical polymerization with a monoacylphosphine oxide, (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (TPO), as a photoinitiator. Gel permeation chromatography (GPC) fractionation of the PS and PMMA samples with initial polydispersities of 1.81 and 2.81, respectively, yielded improved MALDI-TOF MS spectra. Spectral analyses of the PS fractions showed distributions attributable to PS having two diphenylphosphinyl ends and PS having one diphenylphosphinyl end and/or one 2,4,6-trimethylbenzoyl end, indicating that a combination of PS radicals with the highly reactive diphenylphosphine oxide group at one end of the chains was the predominant mode of termination. MALDI-TOF MS results for PMMA fractions provided evidence for termination primarily by disproportionation, but structure determination was confounded by the presence of isobaric peaks. Discernible peaks were obtained by MALDI-TOF MS analyses of GPC fractions of TPO-initiated poly(methyl-d(3) methacrylate-d(5)), in which the major product was PMMA with a diphenylphosphinyl end group and an abstracted deuterium end group, whereas the minor combination product had two diphenylphosphinyl chain ends. (C) 2007 Wiley Periodicals, Inc.