The principal objective of this work was to determine the natural radionuclide contents of different rank coals from the Asturias and Galicia (important mining areas of Northwest Spain). It is known that there is a good correlation between ash content and the natural gamma radioactivity of these coals that may be used as the basis for natural gamma radiometric equipment for on-line determination of their ash content. For a more complete understanding of this correlation, an examination was made of which chemical components of the coal ash have the most significant influence on variations of the natural radionuclide content in coal. One hundred and twenty samples of the different products (run of mine (rom), washed coal, and wastes from coal-washing plants) were analysed for natural radionuclide content, in selected cases for chemical element content of their ashes, and coal wastes for mineral constituents. The radioactive analyses were performed by using a gamma spectrometer with an NaI(T1) crystal. The elemental analyses were by X-ray fluorescence, and the mineral analyses were performed by X-ray diffraction. This study suggests that the highest values of natural radionuclide concentration are in coal wastes. The lignites from Galicia show lower concentration than the coals from Asturias.