We report the potential of Laser-Induced Breakdown Spectroscopy (LIBS) for the determination of lanthanum and yttrium in soils and rocks. Since the main problem to quantify rare earth elements by LIBS is their rich spectra and the consequent frequent spectral interferences with the matrix elements, we demonstrated how thermodynamic modeling of the spectra can assist spectroscopists in the estimation of LIBS sensitivity. The theoretical LOD for La was close to the one retrieved from experimental data (6 ppm), while the theoretical LOD for Y was one order of magnitude higher than the experimental LOD (6 ppm vs. 0.4 ppm). The possible reasons for such a discrepancy are discussed.