The total elemental composition of a sample must be measured or specified for successful application of fundamental parameters type matrix corrections in x-ray fluorescence analyses. Unanalyzed light elements can be dealt with in three ways. They can be related to the concentration of another element by stoichiometry, calculated by difference, or their effects minimized by addition of a diluent of known composition, ‘The relative success of these approaches depends on the composition of the sample. Limiting cases tested are high or low concentrations of analytes having long or short wavelength emission lines, in the presence of additional heavy or light elements. Molybdenum oxide is used as the analyte, with the oxygen serving as a typical unanalyzed light element to be treated by stoichiometry, difference, or dilution. The accuracy and precision of the analysis for molybdenum, using either the Kα or Lα line, is the criterion for judging the success of the strategy. Experimental results and theoretical calculations using the XRF11* matrix correction program are employed. Generally, the difference strategy is inferior to stoichiometry or dilution. However, difference methods can give acceptable results except in the case of a high concentration of short wavelength analyte in a light element matrix. Where stoichiometry is not known, an assumed stoichiometry that is correct within one oxidation number gives results comparable to the difference method.