The pseudomorphic replacement of glass shards by zeolite minerals is a common feature of volcanoclastic sediments. In the majority of cases the initial stage of this reaction is the alteration of the glass surface to a clay mineral or celadonite after which the bulk of the glass is altered to zeolite. This replacement feature is seen particularly well in glass of rhyolitic composition; the zeolite mineral usually being clinoptilolite. Volcanoclastic rocks of Oligocene age exposed in the Rhodope Massif, Bulgaria offered an opportunity to study this reaction experimentally as rocks containing unaltered glass shards are known to be close stratigraphic equivalents of zeolitized tuffs and in this respect are considered to be precursor rocks. Low-temperature hydrothermal reactions conducted on the unaltered glass, which had been clay coated in the laboratory, demonstrates the importance of the clay-glass interface. An hypothesis is put forward to explain this type of zeolitization process and a distinction is drawn between these rocks and other sediments in which zeolite minerals form from volcanic glass without the presence of a clay interface.