Fluidised cracking catalysts, which contain a form of zeolite-Y as the main catalytically active component, are widply used commercially for the conversion of crude oil into more profitable product streams. During the cracking reaction, these catalysts are contaminated with vanadium which has a marked effect on the crystallinity of the zeolite-Y component and, as a consequence, activity and selectivity for hydrocarbon processing is degraded. The purpose of this work has been to carry out a detailed investigation, on the laboratory scale, of the effect of vanadium contaminatioil on both commercial and model rare earth ion-exchanged zeolite-Y catalysts. Vanadium contamination was achieved using a standard (Mitchell) method and catalysts were subject to treatment conditions similar to those found in the regenerator part of a fluidised catalytic cracking unit using a specially constructed furnace. Investigations of the solid state chemical reactions between vanadium and rare earth compounds, both in the presence and absence of silica and alumina support materials typical of those found in commercial catalysts, extend the study. Extensive use is made of magic-angle-spinning nuclear magnetic resonance spectroscopy (5 IV and 27AI), X-ray powder diffraction and surface area measurements for sample characterisation. The apparatus for surface area measurements was constructed during the course of the work. It is suggested that the reduction in crystallinity of rare earth ion-exchanged zeolite-Y in the presence of vanadium is associated with the removal of rare earth ions from the cage structure of the zeolite. The extent of this process depends upon the details of the treatment conditions and important factors are identified. The observation of the formation of LaV04 in a range of hydrothermally treated lanthanum ion-exchanged zeolite-Y samples supports the proposed model.