The technique of high resolution Brillouin spectroscopy has been used to determine the adiabatic elastic constants of single crystals of P-N2 and P-CO as a function of temperature, in an effort to study the a-P phase transition. For all elastic constants, there is an increase of approximately 1 %/K on cooling and there is no evidence of further mode softening, even within 0.5 K of the phase transition. Three large single crystals of a-CO were grown. The orientations were determined, by Laue X-ray diffraction, and correlated to the orientations of the parent P-crystals. The P-phase hexagonal basal planes appear to transform to planes of the form {I 1 0 ) in the a-phase cubic structure.V. ASKARPOUR, H. KIEFTE et M. J. CLOUTER. Can. J. Chem. 66, 541 (1988). Dans le but d'etudier la transition de phase a-P, on a utilise la technique de la spectroscopie de Brillouin 2 haute rCsolution pour determiner les constantes Clastiques adiabatiques de cristaux uniques de P-N, et P-CO en fonction de la temperature. Pour toutes les constantes Clastiques, on observe une augmentation d'environ 1%/K lorsqu'on refroidit; de plus, on ne dCtecte pas d'autre assouplissement de mode, m&me a 0,5 K de la transition de phase. On a fait croYtre trois gros cristaux uniques d'a-CO. On en a determine les orientations par diffraction des rayons-X selon Laue et on a Ctabli un corrClation avec les orientations des cristaux p apparent&. I1 semble que les plans hexagonaux de base de la phase P se transforment en plans de la forme {I I 0 ) lorsqu'on passe B la structure cubique de la phase a .[Traduit par la revue] Introduction Brillouin spectroscopy is a very effective technique for probing the dynamical behaviour of phonons in crystals and lattice instability associated with phase transitions. In particular, it allows for accurate determination of elastic constants in single crystals. The present work constitutes an attempt to use Brillouin spectroscopy (and Laue X-ray diffraction) to study very simple phase transitions in diatomic molecular solids, namely, the ol-P transitions in solids N2 and CO. This represents a continuation of previous Brillouin scattering experiments on single crystals of P-N2 (1) and P-CO (2) very close to their triple points.