We propose an approach for evaluating the efficiency of O2 equilibration between alveolar space and pulmonary capillary blood by applying well-known laws on pulmonary physiology to simultaneous expirograms of O2 and NO. As proof of principle, we assume a normal pulmonary diffusing capacity to reconstruct an exemplary NO time course during normal expiration. On the basis of this analysis we predict the alveolar-mean capillary O2 partial pressure gradient amounting to 3 mmHg for breathing room air at rest, which is smaller than a third of that commonly given. For the purpose of translating our method into practice, one only requires suitable gas analyzers and the appropriate software. Our approach efforts promises to provide more precise evaluation of pulmonary gas exchange in very different study groups, both at rest and during exercise.