Two experiments examined the ability of an added stimulus to interfere with extinction of a target excitatory fear stimulus (a predictor of shock) in human autonomic conditioning. Both experiments demonstrated disruption of extinction when the added stimulus was inhibitory (a predictor of no shock, or safety signal). Subjects showed a return of fear when the target stimulus was tested alone, on both self-reported shock expectancy and skin conductance measures. The second experiment also demonstrated disruption of extinction when the added stimulus was excitatory. This result suggests that protection from extinction may occur even when the added stimulus is not inhibitory. Additional factors that may contribute to protection from extinction include context-specificity, occasion-setting and external inhibition. The results highlight the role that concurrent stimuli play in extinction, and emphasise the need to keep concurrent stimuli as similar as possible to the desired transfer context in practical applications of extinction such as exposure therapy for anxiety.