Using a high-resolution difference-frequency spectrometer, we have studied the shape of four infrared lines of CO, highly diluted in N~. We have approached the analysis from two perspectives. In the first, the spectral profile of an isolated line is empirically represented by a variation from the "standard" model in which the mass-diffusion constant is replaced with a pressureand line-dependent "optical" diffusion constant. In the second, we abandon the concept of an "optical" diffusion constant and replace it with a physically more tenable hypothesis that the departures from the standard model are due to a slight deviation from the usual exponential decay used to describe the collision broadening of spectral lines. This shifts the focus of the discussion from an analysis of the translational dynamics to an analysis of the dynamics of the internal degrees of freedom.PACS number(s): 33.70. Jg, 35.20.Yh, 34.10.+x