Phone: þ31 (0) 30 609 6846, Fax: þ31(0) 30 603 1204Using simultaneous analysis of both the grazing incidence X-ray reflectivity (GIXR) and the angular dependent fluorescence yield from ultrathin layer structures, the densities of thin La and LaN layers of 2-6 nm thickness enclosed by B 4 C layers have been determined with approximately 5% precision. The density of the La layer in these systems is found to be reduced to the bulk La value. This is explained by LaB 6 interlayers formation. The density of LaN layers were similar to the bulk LaN value, which favors this compound as the most energetically stable in LaN/B 4 C layered systems.