We present a line shape analysis of the P(2) and P(7) transitions of CO broadened by Xe in the fundamentalband. The spectra were recorded at 349 K using a difference frequency laser spectrometer. To obtain information on theinfluence of Dicke narrowing, relaxation speed dependence, and line mixing effects, several models for implementation ofDicke narrowing and (or) speed-dependent effects are discussed. From experimental data analysis, we conclude that lineshape models taking into account the Dicke effect only fail in the high pressure regime and lead to optical diffusion pa-rameters that are much larger than the kinetic diffusion ones. On the contrary, a fair interpretation of data is obtained fromspeed-dependent models, so that it is possible to derive a quantitative estimate of optical diffusion effects that appearmuch smaller than the kinetic diffusion ones. Xe-broadening coefficients of CO lines in the fundamental band at 297 and349 K are calculated from a semiclassical formalism involving successively two intermolecular potentials, the atom-atomLennard-Jones model, and a three-term expansion of Legendre polynomials with four adjustable parameters.