Using CO2 for DCBE is recommended as it may decrease pain afterwards but recent studies suggest it produces inferior distension. This prospective double blind study was designed to evaluate the use of an air/CO2 mixture. We randomised 105 patients to receive air, CO2 or a 50/50 mixture as the insufflation agent in DCBE. Gases were administered from prefilled bags. Those requiring additional insufflations before over couch films were recorded. Post procedure symptoms were evaluated by questionnaire. Distension and mucosal coating were assessed independently. There was no difference in mucosal coating. Those given CO2 had significantly less immediate and delayed pain compared to air and less delayed pain compared to the 50/50 mixture. Distension with air was graded better than with the other two agents but the difference did not reach statistical significance. However 50 % of patients receiving CO2 and 40 % of those receiving the mixture required additional insufflation before over couch films as distension was considered suboptimal, compared to 17 % of those given air, which was statistically significant. Our results indicate that using CO2 causes less pain than using air or the mixture although top up insufflations are often required to maintain adequate quality distension.