This randomized, double-blind, placebo-controlled parallel group study will assess the effect on biomarkers measures of cognitive dysfunction, the clinical efficacy and safety of RO4917838 in participants with schizophrenia and schizoaffective disorder. Participants will be randomized to receive either RO4917838 (10 milligrams [mg] daily orally) or placebo for 6 weeks, in addition to their stable antipsychotic medication. Anticipated time on study treatment is 6 weeks.