ABSTRACT: Investigated the psychological refractory period (PRP) effect in a situation involving response uncertainty that must be resolved by decoding each signal. Ss were 40 undergraduates who participated in both Exps I and II. The PRP effect of interference between 2 choice reaction time tasks at short intertask intervals was eliminated when both of the tasks were ideomotor compatible. The PRP effect was, however, obtained when either or both of these tasks were replaced by stimulus-response compatible tasks that were not ideomotor compatible. It is concluded that a major source of the PRP effect is a limited capacity mechanism that (a) translates between an encoded stimulus and a response code, and (b) is not needed when a task is ideomotor compatible.