There is a body of literature dealing with the increased capacity for deception in online environments. This corpus of academic work has relevance for the widespread public concern about the anonymity of the Internet with respect to children who may be contacted by sex offenders. The present paper reports findings from a deception condition study where pairs of subjects engaged in computer-mediated interaction and were asked to evaluate the age and sex of their interlocutors. They were generally successful at this and tended to base their decisions on the content of the conversation. It demonstrates that individuals, despite the anonymity theoretically offered by the Internet, can discern the age and sex of those they are conversing with online, which has implications for police training and practice when engaged in online covert operations.