Contemporary conceptualizations view drug dependence as a "syndrome in which the use of a drug is given a much higher priority than other behaviors that once had higher value" (Jaffe, 1990). Thus, in order to understand drug dependence, the complex interactions among reinforcers must be understood. Consider, for example, the frequently observed case of polydrug abuse. On some occasions, the drugs may be used simultaneously (e.g., cocaine and heroin), whereas on other occasions one drug may be used in lieu of another (e.g., benzodiazepines and opioids). Understanding reinforcer interactions is also vital to developing effective treatment because treatments, be they pharmacological (e.g., methadone, nicotine gum) or nonpharmacological (e.g., AA meetings, alternative behaviors), often try to supplant drug reinforcers with other more acceptable reinforcers. Although understanding reinforcer interactions is important for understanding drug abuse, no method currently exists to quantify or even categorize these...