A perturbative theory has been developed to model the spectral line shapes affected by line-mixing and finite duration-of-collision. The line profile was given in an explicit form tractable for dealing with the line interference and asymmetry problems. The coupling of line-mixing and collision-duration was studied. The simplified model was conditionally applied to the CO2 nu (3) high-frequency wing beyond the bandhead, and was found to reproduce the experimental absorption coefficients well. The effective collision-duration parameters were obtained from the fit of the nu (3) band edge and from the typical CO2-CO2 collision time of 1 ps. These parameters were then used to calculate the Q-branch core region of the CO2 nu (2) band near 667 cm(-1). The coupling effect from line-mixing and finite-duration was theoretically shown to be discernible at the pressure of 1 amagat. This effect grows with increased pressures and, expectedly, becomes significant for higher pressures.