Volatile flavor compounds in ultrahigh-temperature (UHT) processed milk were investigated to determine their role in off-flavor development during aseptic storage. The milk samples were processed at 145 °C for 3 s with and without added ascorbic acid and stored at 3, 22, and 35 °C for 5 months. Flavor isolates were prepared through steam vacuum distillation and subsequent extraction of the distillate with dichloromethane. The isolates were analyzed using gas chromatography and mass spectrometry. The milk was regularly analyzed by various chemical methods and evaluated by a taste panel during storage. Twenty-six compounds were identified, seven of which were not previously reported in UHT milk. Gas chromatographic profiles indicated that 2-pentanone, 2-heptanone, 2-nonanone, and n-hexanal increased most in concentration during storage. The rate of increase in odd carbon-numbered methyl ketones (C3 13) was dependent upon storage temperature, whereas the rate of increase in aldehydes was dependent upon both oxygen content and temperature of storage. Although methyl ketones were the most abundant class of compounds, aldehydes appeared to be most important in contributing to the off-flavor of stored UHT milk.