Phase relations at atmospheric pressure in the pseudobinary join KAlSi₂O₆ (Lc)-NaAlSi₃O₈(Ab) and in the pseudoternary join Lc-Ab-CaAl₂Si₂O₈(An) indicate that leucite is in-compatible with Na-feldspar. In the former join leucite can exist with an alkali feldspar of maximum albite content Ab₅₄. In the Lc-Ab-An join, leucite only coexists with ternary feldspars with high An contents (approximately An₅₀). Under PH₂O conditions leucites may only coexist with alkali feldspars even poorer in Ab than those found at atmospheric pressure. Rare occurrences of coexisting leucite and Na-feldspar in nature have probably not crystallized directly from a melt but may have formed by a process of alkali ion exchange; or they may be unstable assemblages. No support can be found for the suggestions based on thermochemical calculations that albite and leucite are compatible at high temperatures.