Clinical experiences indicate that schizophrenic cognitive disturbances may be partly due to the unsuccessful construction of their cognitive context. In this article, two experiments are introduced in which schizophrenic deficits in utilization of information for construction of effective cognitive context were examined through measurements of event-related potentials, particularly P300s. In a 3-tone discrimination task, schizophrenic subjects failed to elicit P300s to frequent nontarget tones that were as important as standard tones for performing the required task. In the second task, consisting of detecting two consecutive identical tones, unlike healthy controls, schizophrenic subjects failed to display P300s to nontarget tones at target position. These results are discussed in relation to ineffective context construction of schizophrenic subjects. Moreover, a single-trial analysis of relationships between P300s and reaction times in the 3-tone discrimination task disclosed a loose coupling between the stimulus set and response set in patients.