Various amounts of Na2SO4, Na2S2O7, and Na3Fe(SO4)(3) were added to two kinds of coal ash to study the effects of sodium-containing sulfates on ash fusibility. Ash fusion temperature (AFT) tests and X-ray diffraction (XRD) were applied to measure the ash fusibility and detect mineral compositions. Thermodynamic calculations were used to analyze mineral transformations of ash samples. It was found that the AFTs decreased with increasing sodium-containing sulfate contents. The primary sodium-containing minerals in ash mixed with Na2SO4 were nepheline, nosean, and albite, which all had low melting points and easily formed low-temperature co-melts. Labradorite appeared when the ash was mixed with Na2S2O7 and Na3Fe(SO4)(3), leading to lower AFTs when compared to ash mixed with Na2SO4. The trend of liquidus temperatures from thermodynamic calculations was consistent with the AFT tests. The calculations also indicated that sodium-containing minerals were easily formed because of low-reaction Delta G values.