Pebbles from two conglomerates in the Grès du Champsaur formation have been chemically analysed and dated by potassium-argon method. Apart from a few pebbles with diabasic affinities, the rocks are andesites or basaltic andésites with microporphyritic texture. They show signs of alteration or very low grade metamorphism. In one outcrop, there has been extensive calcitization but fresh basic zoned plagioclases are preserved. In the other outcrop, there has been pervasive albitisation and the pebbles are spilites. Including three granitic rocks, twenty two specimens were dated. The results were statistically studied by principal components and cluster analyses. The rocks with diabasic affinities seem older than the andesitic rocks. It appears that in the Champsaur formation these two kinds of volcanic fragments are chronologically distinct. The andesitic pebbles constitute two groups of slightly different ages characterized by a small argon overpressure. Because of this phenomenon, only maximum ages can be derived: 37.6 ±1.3 Ma and 40.2 ±1.3 Ma. Minimum ages cannot be estimated but they must be at least equal to the stratigraphie age, probably uppermost Eocene. A currently admitted age for the Eocene-Oligocene boundary is 35.7 Ma. If the andesite flows are coeval with the sedimentation, the volcanic zone should be looked for in the vicinity of the sedimentation basin of the Champsaur formation; if they are older, the volcanics might have originated in a more internal zone and then transported as part of a nappe to the rim of the sedimentation basin. We do not have, as yet, a sufficient number of data to decide between the two hypotheses. More age determinations are needed with improvements of the statistical methods.