The vibrational energy relaxation has been studied for hydrogen chloride diluted in liquid and solid xenon near the fusion point. The v=1 relaxation time is measured in the liquid and in the solid vs concentration. On solidification of the xenon, we observe the continuity of the relaxation times for HCl isolated in the solution. This behavior, different from that observed for vibrational spectroscopy, points out the minor role played, for the relaxation behavior, by the symmetry of the site occupied by the hydrogen chloride molecule. In the solid phase, we measured the relaxation times for the first three excited vibrational states vs temperature up to the fusion point. After considering the importance of anharmonic coupling in the vibrational interaction for this system, we can understand our results in terms of binary approaches to vibrational relaxation currently applied to the liquid phase, thus underlying the unity of the relaxation process in the gas, liquid, and solid phases.