The diffusing capacity of the lung, DL, is a critical physiological parameter, yet the currently accepted clinical model (Jones-Meade) assumes a well-mixed alveolar region, and a constant DL independent of alveolar volume, VA, despite experimental evidence to the contrary. We have formulated a new mathematical model [Tsoukias, N.M, Wilson, A.F., George, S.C., 2000. Respir. Physiol. 120, 231-249] that considers variable alveolar mixing through a single parameter, k (0<k<1), and a DL that is a positive function of VA (DL=a+bVA or DL=alphaVA(beta)). The goal of this study is to determine the suitability of this model to determine the unknown parameters a, b, alpha, beta, and k from experimental data in normal subjects. The model predicts that the normal lung fills, in part, sequentially (k=0.51+/-0.35). The following average values in all seven subjects were obtained: DLNO=48.VA(2/3) ml/min/mmHg and DLCO=20+0.7.VA ml/min/mmHg (STPD) where VA is expressed in L (STPD). We conclude that the mathematical model is suitable for identifying the unknown parameters and thus can be used to characterize the degree of alveolar mixing (or sequential filling) as well as the volume dependence of DL.