Frequency assignment procedures in mobile radio systems generally aim at achieving a measure of intermodulation compatibility, and, in principle, provided sufficient numbers of channels are available and may be assigned with the minimum of restrictions, then this objective is achievable (at least as far as third-order products are concerned). However, in cellular systems, additional constraints are imposed by the need to multicouple cell centre base station transmitters on to a common an tenna, and this requirement sets a lower limit on the minimum frequency separation at cell centres, which, in turn, makes achievement of third-order compatible frequency plans extremely difficult. The paper examines the relationship between the minimum frequency separation for third-order compatibility and the number of available channels and cell cluster size, in order to identify optimum configurations.