AbstractA mathematical model is derived for tertiary X‐ray fluorescence from heterogeneous samples based on generally accepted primary X‐ray fluorescence models. Variable source‐sample‐detector geometry, sample particle size and packing fraction effects are included. The model is expressed explicitly for three different ways of treating the attenuation within the sample. Calculations of the model are made for comparison with primary and secondary fluorescence calculations, and for comparison with calculated and experimantal results reported in the literature which include tertiary