Matrix-assisted laser desorption/ionization time-of-flight (MALDI/TOF) mass spectrometry has been used for the analysis of poly(butyl methacrylate) synthesized by group-transfer polymerization, and the absolute molecular weight distributions have been determined for polymers up to 100 000. For lower molecular weight poly(butyl methacrylate) we obtained structural information confirming composition and end groups. The compounds were desorbed from trans-3-indoleacrylic acid and detected as positive ions showing molecular weight and several multiply charged distributions from submilligram amounts of sample. The results provided answers to ambiguities which two separate GPC measurements had created. MALDI results were supported by light scattering measurements.