Continuous-flow isotope ratio mass spectrometry (CF-IRMS) has been shown to minimize sample preparation and reduce the sample size requirements and time for the determination of carbon and nitrogen. An inexpensive CF inlet was developed for the determination of the 18O isotopic abundance in CO2. Initial efforts were hampered by significant exchangeable oxygen within the inlet system, which was minimized by design changes and post-analysis mathematical correction. The system was tested for accuracy in the measurement of 18O abundance in standards and was accurate to 0.04/1000 with a precision of 0.03/1000. Additional tests were performed using urinary water for the measurement of energy expenditure by the doubly labeled water method and the results agreed to within 1% with those from traditional off-line, dual-inlet IRMS. The CF-IRMS reduced the analysis time to 2 min per sample compared with 15 min for traditional duel-inlet IRMS.