Summary: The composition of the various phases present in the rhönite-bearing leucite basanites of the southern part of the Sillon Houiller district have been determined with the electron microprobe. Clinopyroxenes show strong progressive enrichment in Al2O3 and TiO2, maximum values being 13 % and 6 % respectively. These extreme compositions are comparable to others found in feldspathoidal lavas from other provinces, and can be explained by the low silica content of the liquids they crystallized from, and high cooling-rates. Rhönite composition is similar to those previously determined in various terrestrial occurrences. The chemical compositions of the lavas differ from those of basanites, and are intermediate as far as Ca, A1, and Si contents between basanites and melilite-bearing basic lavas. The presence of rhönite can be explained by bulk composition, but is not a consequence of a particularly high Fe and/or Ti content. Xenoliths are absent from these lavas, so that there is no petrographic indication of possible contamination. It is tentatively concluded that the high K/Na ratio is primary.