The objectives of this research were to study the effects of high intensity (0.5, 0.75, and 1.0 mW/cm (2)), dose (0.5, 1.0, and 1.5 J/cm (2)), and postharvest time (1 and 4 days) on the vitamin D 2 formation in Portabella mushrooms ( Agaricus bisporus) as a result of UV-B exposure, as well as the vitamin D 2 degradation in treated mushrooms during storage. Within each intensity application, dose had the largest effect where more exposure converted more vitamin D 2 from ergosterol. Similar dose across each intensity application resulted in similar vitamin D 2 concentration. Practical commercial production requires as short a treatment time as possible, and intensity was a major factor from this standpoint where the time it took to achieve a similar vitamin D 2 concentration for similar dose exposure was significantly reduced as intensity increased. By using an intensity of 1.0 mW/cm (2) at a dose of 0.5 J/cm (2), the concentration of vitamin D 2 produced was 3.83 microg/g dry solids of mushrooms in 8 min, whereas using an intensity of 0.5 mW/cm (2) at a dose of 0.5 J/cm (2), the concentration of vitamin D 2 produced was 3.75microg/g dry solids of mushrooms in 18 min. Also, postharvest time did not have a significant effect on vitamin D 2 formation in mushrooms that were treated 1 and 4 days after harvest. Vitamin D 2 degraded in treated mushrooms during storage by apparent first-order kinetics, where the degradation rate constant was 0.025 h (-1). The information provided in this study will help mushroom producers develop commercial-scale UV treatment processes to add value to their crop while improving consumer health.