This study sought to objectify the distinction between schizophrenia and schizoaffective disorder in terms of standard tasks measuring verbal and non-verbal cognitive ability, auditory working memory, verbal declarative memory and visual processing speed. Research participants included 103 outpatients with a diagnosis of schizophrenia, 48 with schizoaffective disorder, and 72 non-patients from the community. Schizophrenia patients were impaired on all cognitive measures relative to schizoaffective patients and non-psychiatric participants. Regression-based prediction models revealed that cognitive measures classified schizophrenia patients accurately (91%), but not patients with schizoaffective disorder (35%). In addition, there was no statistical evidence for the unique predictive validity of any specific cognitive task. Patients with schizophrenia were significantly more symptomatic and had greater community support requirements than those with schizoaffective disorder. However, group differences in cognitive performance are insufficient to separate these syndromes of psychotic illness.