The purpose of this study was to test the accuracy of a new portable spirometric-telemetric device (Oxycon Mobile) over a wide range of exercise intensities against an accurate stationary apparatus (Oxycon Pro). Therefore, fifteen endurance-trained subjects (VO2peak: 58.8 +/- 5.2 ml x min(-1) x kg(-1)) performed an incremental as well as an endurance exercise test on two different occasions. Gas exchange variables were measured by the two ergospirometric devices simultaneously. For this purpose, a special face mask was designed which allowed respiratory gas sampling with both devices at exactly the same time. Compared to the Oxycon Pro as an accurate reference system, the Oxycon Mobile showed significantly lower values for VO2 at 200 and 250 W during the incremental exercise test. For VCO2 no significant differences were found although the Oxycon Mobile seemed to systematically measure higher values. With regard to RER the Oxycon Mobile showed significantly higher values at all workloads tested. We therefore dissuade from comparing gas exchange variables collected by the Oxycon Mobile with data measured under laboratory conditions by an accurate reference system like the Oxycon Pro. Furthermore, the extrapolation of information about substrate utilization during different exercise durations and intensities from data measured by the Oxycon Mobile seems inappropriate.