["Journal of Investigative Psychology and Offender Profiling, Volume 23, Issue 3, October 2026. ", "\nABSTRACT\nWhether an alibi corroborator willingly cooperates with police can affect views of the defendant and alibi, but cooperativeness has only been examined in one past alibi study. Participants may infer that cooperative corroborators are more truthful than uncooperative corroborators, and these views may be impacted by the participants' general attitudes towards police. The corroborator's certainty that they were with the defendant also may vary under repeated questioning. The consistency of corroborator certainty under repeated questioning has yet to be examined in the alibi context. Online U.S. participants from CloudResearch's Mechanical Turk Toolkit (N = 320) took on the role of mock jurors and evaluated a mock case where corroborator cooperativeness (cooperative vs. uncooperative) and the consistency of his certainty (maintained high, maintained low, strengthened, weakened) varied between conditions. Cooperative (vs. uncooperative) and certain (vs. uncertain) corroborators led to higher alibi believability and lower likelihood of guilt ratings and more positive views of defendant and corroborator character traits. More positive police attitudes and greater likelihood of reporting to police were correlated with lower alibi believability ratings. Analyses of an open‐ended question about the corroborator's behaviour with police showed that participants included a range of explanations for his behaviour. Implications and future directions are discussed.\n"]