Ozone cracking and flex cracking of crosslinked elastomer blends of brominated isobutylene/para-methylstyrene copolymer (BIMSM) and unsaturated elastomers, such as polybutadiene rubber (BR) and natural rubber (NR), are studied. This saturated BIMSM elastomer, which is a terpolymer of isobutylene, para-bromomethylstyrene, and para-methylstyrene, functions as the ozone-inert phase of the blend. Ozone cracking is measured by the failure time of a tapered specimen under a fixed load in a high severity ozone oven, whereas flex cracking is ranked by the De Mattia cut growth. The ozone resistance of BIMSM/BR/NR blends is compared to that of a BR/NR blend (with or without antiozonant) at constant strain energy densities. The effects of the BIMSM content in the blend, the structural variations of BIMSM, and the network chain length between crosslinks. on these two failure properties, which are important in crosslinked compounds for applications in tire sidewalls, are discussed. (c) 2006 Wiley Periodicals, Inc. I Appl Polym Sci 103:2183-2196,2007