The apparent bond strength of some composite solid-rocket propellants to elastomeric substrates is governed by the propellant cohesive strength. When these propellants are tested (whether by the tensile or peel method), failure is invariably cohesive within the propellant near the interface. Addition to the propellant of substances expected to lower its cohesive strength (e.g., substrate ingredients such as cure inhibitors and plasticizers) decreases the apparent propellant-substrate bond strength and lowers the propellant Rex hardness. No correlation was found between apparent bond strength and the relationship of propellant binder surface tension to substrate critical surface tension.