Relative Sensitivity Factors (RSFs) for the rare earth elements Ce, Nd, Sm, Eu, Gd, Dy, Er, and Lu have been determined in U3O6-graphite and pure graphite matrices under similar experimental conditions employing Spark Source Mass Spectrometry (SSMS) with electrical detection system in magnetic peak switching mode. The RSF values differ very much when the matrix element (uranium in U3-graphite system or carbon in pure graphite) is the reference, But they show very good agreement when Er, one of the rare earth elements is chosen as internal standard. The literature data on RSF values for the rare earth elements in Y2O3as well as Al matrices are compared with those obtained in the present work. An attempt has been made to explain the similar trend in the RSF values in all these Matrices in terms of the decomposition pattern of the rare earth oxides. An empirical equation relating the RSFs with the decomposition energies of the lanthanide sesquioxides and monoxides has been shown to be useful in computing the RSFs for the lanthanides in various matrices.