The decomposition of poly(methyl methacrylate) (PMMA) under the action of 248 nm, 16 ns laser pulses at 250 mJ/cm(2) was studied in vacuum. A sensitive electron impact ionizer/quadrupole mass spectrometer system was used to measure mass spectra and time-of-flight (TOF) distributions of the neutral photoproducts. The incubation and photoablation reactions were unambiguously resolved in the TOF distributions. The incubation reaction produces slow-moving species with mass peaks at m/e = 15, 28, 29, 30, 31, 32, 59, and 60 which can be assigned to methanol, CO, and methyl formate. These products are consistent with the incubation mechanism proposed by Stuke and co-workers, namely, single-photon absorption at the carbonyl side-chain chromophore followed by side-chain scission. The incubation reaction produces a partially unsaturated and more strongly UV-absorbing polymer backbone. The ablation of this degraded material at 250 mJ/cm(2) produces mainly species with molecular weights < 150 and translational energies around 0.3 eV. The relationship of these results to an extensive and contradictory body of data in the literature is discussed.