The profiles of the quadrupole-induced transitions (S1(0), S1(1)) of the pressure-induced fundamental infrared absorption band of hydrogen are studied in H2–Ar, H2–Kr, and H2–Xe gas mixtures up to high densities a few degrees above the critical temperature of the rare gas and in the corresponding liquid mixtures along the liquid–vapor line. The half-width δq of the transitions remains practically constant for the gas mixtures up to densities of several hundred amagat and then decreases rapidly. Values of δq for the liquid mixtures, when corrected for the temperature difference, are the same as the gas values at the same density. The narrowing of the transitions in the high-density region is interpreted in terms of the diffusional narrowing theory of Zaidi and Van Kranendonk, and is used to calculate diffusion coefficients D12 of H2 molecules in dense rare gas fluids. The results are in reasonable agreement with values of D12 previously deduced from the intercollisional interference effect in the overlap-induced components (Q1(0), Q1(1)) of the same band.