Zeolitic tuffs from the Xerovouni location of the Avdella area (Evros Prefecture, Greece) contain on average 54 wt.% HEU-type zeolite, 6 wt.% smectite, 4 wt.% mica (64 wt.% microporous minerals), 8 wt.% alkali feldspar, 9 wt.% plagioclase, 11 wt.% cristobalite and 8 wt.% quartz (36 wt.% non-microporous minerals). The chemical formula of the HEU-type zeolite is Ca2.4K0.6Na0.4Mg0.3Al6.7Si29.3O72·17H2O and the tuffs contain on average 72.3 wt.% SiO2, 11.9 wt.% Al2O3, 1.2 wt.% Fe2O3, 1.0 wt.% MgO, 2.8 wt.% CaO, 1.3 wt.% Na2Oand 1.9 wt.% K2O. The zeolitic tuffs show an average ammonium exchange capacity of 144 meq/100 g. HEU-type zeolite accounts for the most of the ammonium exchange capacity, while smectite and mica contribute to a relatively small extent. The ammonium exchange capacity of the Xerovouni zeolitic tuffs showed positive correlations with (a) the HEU-type zeolite content, (b) the total microporous minerals content, (c) the loss on ignition content and (d) the CaO content. Such materials could be used in a large variety of agricultural, aquacultural, industrial and environmental applications.