A method for evaluating the sodium tolerance of FCC catalysts by use of a cyclic deactivation unit (CDU) has been developed, In the CDU sodium is impregnated on the catalyst through the introduction of sodium naphthenates into the feedstock during the cracking step. The sodium tolerance of commercial FCC catalysts was studied using this method. Both the amount of added sodium and vanadium were varied to study the effect of increasing metal loading. The deactivated samples were characterised (nitrogen adsorption, elemental analysis, XRD) and tested in MAT. The effect of increasing amounts of sodium on the cracking behaviour of the FCC catalysts was very small compared to the effect of increasing amounts of vanadium. Sodium deposition resulted in a lower activity and less hydrogen transfer reactions, leading to more olefins and less paraffins in the gasoline product. For the commercial FCC catalysts at least 4500 ppm sodium at a level of about 2-2500 ppm vanadium could be applied without changing the cracking characteristics significantly, or without a break-down of the catalyst structure.