The use of covert human intelligence sources for the protection of national security is an important but under-researched area. In light of increased globalisation, and technological developments, examining covert intelligence gathering online may have operational relevance. This study examined the use of similarities in self-disclosure to develop rapport with mock covert human intelligence sources online. A sample of 66 participants were asked to assume the role of a CHIS, listen to an audio recording of a meeting of the criminal network they are part of, and strike a balance between being forthcoming to their handler while at the same time not revealing all information they possessed. They were then interviewed online by a mock handler who opened the conversation in one of three ways: self-disclosing information that highlighted similarities between herself and the CHIS, selfdisclosing information that highlighted dissimilarities, or control (no disclosure). Participants completed a follow-up rapport questionnaire based on their interaction with the handler. The study found that mock sources rated rapport higher in the similarities condition in comparison with other conditions; however, it did not translate into a significant difference in the intelligence reported. The findings of this research and its policy and practice implications are discussed.