Diagenetic alteration in two tuff horizons (the lower and upper tuff units) in the Gördes region of Turkey led to the formation of mainly heulandite-clinoptilolite-type zeolites and also clay and silica minerals and K-feldspar. Possible variations in the climate and geological environment followed by variations in the hydrological system and the composition of the Miocene lake water and groundwater at that time resulted in the mineralogical facies. The lower tuffs are unaltered or slightly altered in the northern part and are more altered, particularly to zeolite (clinoptilolite) in the southern part of the study area. Heulandite and K-feldspar are mainly authigenic minerals in the upper tuff unit. The ammonium-exchange capacities of the 16 samples were investigated to understand the effects of the mineralogical composition on their ion-exchange capacities, which vary in the range 0.19-2.00 mEq/g. It is observed that the ion-exchange capacities are strongly dependent on the zeolite contents of the rocks. The correlation coefficient of this linear relation is 0.86. The most significant increase in ammonium-exchange capacity (from 0.2 to ∼1.0 mEq/g) was observed when the zeolite contents increased from 0 to 30 wt.%. The second increase in exchange capacity was observed for samples containing >80 wt.% zeolite. Although there is no significant effect from the other authigenic minerals, smectite has a positive effect, and K-feldspar and opal-CT have almost no effect on the ion exchange capacities. © 2005 The Mineralogical Society.