Alteration of glassy andesite rocks was studied by detailed lithological and textural analyses in Neresnica Formation near Zvolen. Products of this alteration were investigated by X-ray diffraction, IR-spectroscopy, TEM and chemical analyses. Lithological and textural phenomena indicate syngenetic alteration in aquatic environment producing mixture of halloysite, mixed layer illite/smectite, kaolinite (halloysite)/smectite and amorphous iron.