50 alkali feldspars from a Variscan porphyritic granite and 27 from two country rocks of Caledonian and Variscan age respectively have been investigated for their bulk chemistries and their structural states. Ordering is illustrated in thebc-plot afterStewart andWright (1974); unmixing is illustrated in a newly designed diagram with the Or-contents of the bulk feldspar and that of the unmixed K-rich phase on the coordinates. The feldspars from the granite have a greater variability with regand to chemistry, degree of ordering and degree of unmixing than those of the country rocks. Ordering and unmixing depend mainly on the action of a hydrous fluid phase. During the in situ formation of the granite, water must have accumulated in the granitic magma and must have remained in the granite after consolidation. Temperatures derived by means of the two feldspar geothermometer indicate selective loss of sodium during the postmagmatic stage and before unmixing of the alkali feldspars. Loss of sodium makes unmixing increasingly sluggish.