Publisher Summary This chapter discusses the results of weathering exposure of butyl rubber, bromobutyl rubber, and chlorobutyl rubber. Butyl rubber is a copolymer of isoprene (minor) and isobutylene. Exxon bromobutyl (BIIR), a brominated copolymer of isobutylene and isoprene, has a predominantly saturated backbone of butyl. Exxon chlorobutyl (CIIR), a chlorinated copolymer of isobutylene and isoprene, is an elastomeric isobutylene-isoprene copolymer (halogenated butyl) containing reactive chlorine. Chlorobutyl and bromobutyl contain 1.2% of the halogenated butyl. The halogen addition changes the cure characteristics and can improve adhesion. Butyl, bromobutyl, and chlorobutyl are resistant to aging, ozone, and weathering from atmospheric exposure. Carbon black-filled bromobutyl compounds have high resistance to weathering. Isobutylene backbone polymers are generally susceptible to attack by UV radiation. In lightcolored compounds, this results in chain scission and the development of surface tack with high dirt retention. Carbon black-filled isobutylene polymer compounds have good weathering resistance to UV attack and weathering.