When the doubly-labeled water (DLW) method is used to measure total daily energy expenditure (TDEE), isotope measurements are typically performed using isotope ratio mass spectrometry (IRMS). New technologies, such as off-axis integrated cavity output spectroscopy (OA-ICOS) provide comparable isotopic measurements of standard waters and human urine samples, but the accuracy of carbon dioxide production (VCO₂) determined with OA-ICOS has not been demonstrated. We compared simultaneous measurement of VCO₂ obtained using whole-room indirect calorimetry (IC) with DLW-based measurements from IRMS and OA-ICOS. 17 subjects (10 female; 22 to 63 yrs.) were studied for 7 consecutive days in the IC. Subjects consumed a dose of 0.25 g H₂¹⁸O (95% APE) and 0.14 g ²H₂O (99.8% APE) per kg of total body water, and urine samples were obtained on days 1 and 8 to measure average daily VCO₂ using OA-ICOS and IRMS. VCO₂ was calculated using both the plateau and intercept methods. There were no differences in VCO₂ or TDEE measured by OA-ICOS or IRMS compared with IC when the plateau method was used. When the intercept method was used, VCO₂ measured using OA-ICOS did not differ from IC, but VCO₂ measured using IRMS was significantly lower than IC. Accuracy (~1-5%), precision (~8%), intraclass correlation coefficients (R=0.87-90), and root mean squared error (30-40 L/day) of VCO₂2 measured by OA-ICOS and IRMS were similar. Both OA-ICOS and IRMS produced measurements of VCO₂ with comparable accuracy and precision when compared to IC.