The clay minerals associated with zeolitization of perlites in endogenous domes in the eastern part of the Borovitza caldera are: illite-1Md formed by the alteration of the glass in the perlite-trachyrhyolite transition zone, and illite/smectite mixed-layers in zeolitized perlites from the dome periphery. The illite is ferriferous (0.94–1.05 Fe³⁺) and has a high K content (0.69–0.76) characteristic of intermediate interlayer charge. It is associated mainly with K-mordenite, quartz and adularia. The I/S has from 90 to 10 smectite layers (estimation from chemical analyses) which are montmorillonitic and only rarely beidellitic. They are associated with clinoptilolite, opal-cristobalite, adularia and mordenite. This mineralogical zonation is related to decreasing temperature and increasing alkalinity of the solutions from the transition zone to the peripheral perlitic one. The chemical reactions established from microprobe analyses give evidence that the hydrothermal alteration of perlites is not isochemical, and that Fe, Mg and Ca have to be imported while alkali cations, mainly Na are lost.