The separation of coal and pyrite particles by selective agglomeration with oil can be troublesome for coals which are weakly hydrophobic or hydrophilic and, therefore, are not strongly agglomerable. The hydrophobicity of lower rank coals and other coals which have become oxidized is generally low. On the other hand, the hydrophobicity of oxidized pyrite can be appreciable. The problem of separating such coals from pyrite was studied by oxidizing a moderately hydrophobic coal with air at 150°C for either 8, 20, or 72 hr to systematically reduce its hydrophobicity. Samples of the raw and treated coal were subsequently mixed with aged mineral pyrite from Peru and agglomerated with heptane. Although a mixture of raw, unoxidized coal and pyrite was largely separated by this method, it proved increasingly difficult to separate the materials as the coal became increasingly oxidized. While raw coal was agglomerated in preference to pyrite, this preference changed as the coal was oxidized. After 20 hr. of oxidation, coal and pyrite were agglomerated almost to the same extent. Finally after 72 hr. of oxidation, more pyrite was agglomerated than coal. This trend was reversed by adding thioglycolic acid (TGA) to suppress the agglomeration of pyrite. Therefore, by conducting agglomeration in the presence of 0.002 M TGA, the separation of coal oxidized for 20 hr from pyrite was nearly as good as the separation of the unoxidized coal and pyrite without TGA.