The volume of oxygen exchanged at the mouth during a breath (V_O2,m) is equal to that taken up by pulmonary capillaries (V_O2,A) only if lung oxygen stores are constant. The latter change if either end-expiratory lung volume (EEV_L), or alveolar oxygen fraction (F_AO2) change. Measuring this requires breath-by-breath (BbB) measurement of absolute EEV_L, for which we used optoelectronic plethysmography (OEP) combined with measurement of F_O2 at the mouth to measure V_O2,A = V_O2,m - (EEV_L*F_AO2 + EEV_L*F_AO2), and divided by respiratory cycle time to obtain BbB oxygen consumption (V'_O2) in 7 healthy men during incremental exercise and recovery. To synchronize O₂ and volume signals we measured gas transit time from mouthpiece to O₂ meter, and compared V'_O2 measured during steady-state exercise using expired gas collection with the mean BbB measurement over the same time period. In one subject we adjusted the instrumental response time by 20 msec increments to maximize the agreement between the two V'_O2 measurements. We then applied the same total time delay (transit time plus instrumental delay = 660 msec) to all other subjects. The comparison of pooled data from all subjects revealed r²=0.990, percent error = 0.039±1.61SE and slope=1.02±0.015SE. During recovery increases in EEV_L introduced systematic errors in V'_O2 if measured without taking EEV_L^.F_AO2+EEV_L^.F_AO2 into account. We conclude that OEP can be used to measure breath-by-breath V'_O2 accurately when studying BbB gas exchange in conditions when EEV_L changes as during on- and off-transients.