We present high density experimental and theoretical results on CO₂–He absorption in the v₃ and 3v₃ infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. Computations are based on an impact line-mixing approach in which the relaxation operator is modeled with the energy corrected sudden (ECS) approximation. Comparisons between experimental and calculated results demonstrate the accuracy of the ECS approach when applied to band wings and band centers at moderate densities. On the other hand, small but significant discrepancies appear at very high pressures. They are attributed to a number of reasons which include nonlinear density dependence due to the finite volume of the molecules, neglected contributions of vibration to the relaxation matrix, and incorrect modeling of interbranch mixing.