One problem in coal gasification is the release of alkali species, which can cause fouling and corrosion. Depending on their composition, slags have a high potential for alkali retention. Therefore, the vapor pressures of alkali species (Na, K) over four real solidified gasifier slag samples were determined over the temperature range 1134 to 1591 K by Knudsen effusion mass spectrometry (KEMS). In addition, the residues from the KEMS investigations were studied by X-ray diffraction (XRD) and scanning electron microscope/energy dispersive X-ray (SEM/EDX) to determine the phase composition of the samples and map the alkali distribution. A strong correlation between the composition of the slags in terms of amount of acidic oxides (SiO2, Al2O3) and basic oxides (CaO, FeO) and the volatility was determined. The volatility results showed that higher acidic oxide content leads to higher alkali retention capability. The XRD and SEM/EDX analysis of the KEMS residues showed that an anorthite type phase contains more Na than all the other phases in the equilibrium.