The rare earth elements (REE) together give guite complex X-ray emission spectra with a considerable number of overlaps at analytical energies by lines of other REE with lower atomic numbers. Where the concentration of REE is high, as in lanthanide minerals, this Interference is more difficult to rectify. Smith and Gold resolved a similar problem with lower atomic number elements in energy dispersive microprobe analysis by establishing a series of overlap coefficients. They asserted that accurate corrections were necessary because of the relatively poor overall resolution of the instrument and that these should not be limited to the major coincidences. Some of the smaller values had probably been ignored because they were considered statistically Insignificant. The mathematical matrix of Smith and Gold covered 22 elements from fluorine to barium, with intensity coefficients (other than intraelement) quoted from 0.01% to 282.1% and with ZAF corrections necessary in cases of K to L conversion. The overlap coefficients were also adjusted for matrix effects.