Flash pyrolysis of monodispersed polyethyleneglycol (PEG) with Mw = 2090 was conducted at 370-550 degrees C and 2 and 11 atm Ar by T-jump/FTIR spectroscopy. The overall decomposition process is endothermic. A new approach to determine the composition of the vaporized product mixture is described in which partial. least-squares optimization and quadratic programming were used to determine the degree of polymerization (DPn) of oligomer classes, and to resolve the concentrations of the individual products in the pyrolysis gas mixture. The behavior of nine individual products and classes of products is described as a function of temperature, time, and pressure. A significant change in the relative mole fractions occurred in the 450 +/- 30 degrees C range when flash pyrolysis was conducted at 2 atm. Below this temperature range, volatile products and a glassy residue form from the parent polymer; DPn less than or equal to 1.7. Above this temperature range, volatile products from both the parent polymer and the glassy residue dominate; DPn similar or equal to 2.5. Aldehyde-containing products form only from the parent polymer and probably originate from the end chain -OH groups. Averaged over the temperature range studied, C-O and C-C homolysis occur approximately equally. The ratios of the volatile products from flash pyrolysis at 11 atm Ar are in line with normal expectations, in that products with lower molecular weights dominate at higher temperatures.