The purpose of this study was to evaluate the psychometric properties of an audio computer-assisted self-interviewing Web-based software application called the Social Information Processing Application (SIP-AP) that was designed to assess social information processing skills in boys in 3rd through 5th grades. This study included a racially and ethnically diverse sample of 244 boys ages 8 through 12 (M = 9.4) from public elementary schools in 3 states. The SIP-AP includes 8 videotaped vignettes, filmed from the first-person perspective, that depict common misunderstandings among boys. Each vignette shows a negative outcome for the victim and ambiguous intent on the part of the perpetrator. Boys responded to 16 Web-based questions representing the 5 social information processing mechanisms, after viewing each vignette. Parents and teachers completed measures assessing boys' antisocial behavior. Confirmatory factor analyses revealed that a model positing the original 5 cognitive mechanisms fit the data well when the items representing prosocial cognitions were included on their own factor, creating a 6th factor. The internal consistencies for each of the 16 individual cognitions as well as for the 6 cognitive mechanism scales were excellent. Boys with elevated scores on 5 of the 6 cognitive mechanisms exhibited more antisocial behavior than boys whose scores were not elevated. These findings highlight the need for further research on the measurement of prosocial cognitions or cognitive strengths in boys in addition to assessing cognitive deficits. Findings suggest that the SIP-AP is a reliable and valid tool for use in future research of social information processing skills in boys.