We have developed a photosensitive system having low dielectric constant with mesoporous. The photosensitive system we developed is patternable and the mesoporosity is generated through the combination of photo exposure and thermal treatment. The mesopores were formed in a photosensitive material to reduce its dielectric constant. Tert-butoxycarbonyl (t-BOC) containing acrylic copolymer was activated as a photosensitive material via photochemical reactions. Iodonium salt as a photo acid generator was exposed to ultraviolet light with a wavelength of under 365 nm to form the corresponding Lewis acid. The side chains of t-BOC were cleaved by this Lewis acid to yield isobutylene and acrylic acid groups. The small molecules thus formed were further heated in the polymer matrix to generate mesopores. Notably, the t-BOC content and heat affect the dimensions and number of mesopores. The dielectric constant decreased as the density of mesopores increases. The formation of mesopores was observed by transmission electron microscope and scanning electron microscopy. We have also studied the mechanisms of formation of mesopores and their effects on the dielectric constant. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 3269-3277, 2013