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Subvolcanic phonolite intrusions of the Kaiserstuhl Volcanic Complex (Germany) show variable degrees of alteration. Their secondary mineralogy has been characterized by petrographic textural observations, bulk-rock powder X-ray diffraction, thermogravimetry, differential thermal analysis, and electron probe microanalysis. The alteration assemblage is dominated by various zeolites that occur in fissures, vugs, and as replacement products of primary phases within the phonolite matrix. Phonolites in the eastern Kaiserstuhl were emplaced into a sedimentary sequence and are characterized by high zeolite contents (Endhalden: 48 wt%, Fohberg: 45 wt%) with the temporal sequence: ± thomsoniteCa ± mesolite -- gonnardite -- natrolite -- analcime. In the western Kaiserstuhl zeolite contents are lower (Kirchberg: 26 wt% or less) and the crystallization sequence is: ± thomsonite-Ca -- gonnardite -- natrolite -- chabazite-Ca. Pseudomorphic replacement textures and barite inclusions in secondary aggregates suggest that zeolites grew at the expense of a sulfate-bearing sodalite-group mineral, i.e., haüyne. Fresh grains of sodalite-haüyne are only found at Kirchberg, whereas the pervasive alteration at Fohberg and Endhalden transformed feldspathoid minerals completely to zeolites. [ABSTRACT FROM AUTHOR]