The excellent chemical properties and resistance to oil of poly(acrylonitrile-co-butadiene), or nitrile-butadiene rubber (NBR), has led to the extensive use of these elastomers as O-ring material in the oil extraction industry. The degradation of NBR gaskets is known to occur during the well completion and oil extraction process when they are exposed to bromide fluids such as ZnBr2-based completion fluid. Samples of NBR, polyacrylonitrile, and polybutactiene were exposed to ZnBr2-based completion fluid and analyzed by attenuated total reflectance (ATR) and diffuse reflectance IR. These analyses showed that the ZnBr2 completion fluid promoted the hydrolysis of the nitrile group to form amides and carboxylic groups. The carbon-carbon double bonds in NBR were unaffected after short exposure to ZnBr2-based completion fluid but were quickly hydrated in acidic bromide mixtures. (C) 2003 Wiley Periodicals, Inc.