Purpose: Automated metabolic analyzers are frequently utilized to measure maximal oxygen consumption ($\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$). However, in portable devices, the results may be influenced by the analyzer’s technological approach, being either breath-by-breath (BBB) or dynamic micro mixing chamber mode (DMC). The portable metabolic analyzer K5 (COSMED, Rome, Italy) provides both technologies within one device, and the authors aimed to evaluate differences in $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ between modes in endurance athletes. Methods: Sixteen trained male participants performed an incremental test to voluntary exhaustion on a cycle ergometer, while ventilation and gas exchange were measured by 2 structurally identical COSMED K5 metabolic analyzers synchronously, one operating in BBB and the other in DMC mode. Except for the flow signal, which was measured by 1 sensor and transmitted to both devices, the devices operated independently. $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ was defined as the highest 30-second average. Results: $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ and $\dot{\mathrm{V}}{\mathrm{CO}}_2@\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ were significantly lower in BBB compared with DMC mode (−4.44% and −2.71%), with effect sizes being large to moderate (ES, Cohen d = 0.82 and 1.87). Small differences were obtained for respiratory frequency (0.94%, ES = 0.36), minute ventilation (0.29%, ES = 0.20), and respiratory exchange ratio (1.74%, ES = 0.57). Conclusion: $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ was substantially lower in BBB than in DMC mode. Considering previous studies that also indicated lower $\dot{\mathrm{V}}{\mathrm{O}}_2$ values in BBB at high intensities and a superior validity of the K5 in DMC mode, the authors conclude that the DMC mode should be selected to measure $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ in athletes.