Studies about microwave imaging systems require complex models of biological materials to test the performance of prototypes in realistic situations. Tissue phantom materials have been studied and measured in order to determine their suitability to simulate biological substances around 2.45 GHz. The paper presents sorne dielectric constant data of tissues and phantom materials.A permitivity measurement system,based on an open-ended coaxial line, is also presented. Several materials and mixtures for phantom construction have been cbaracterized using this method,and the relevant measurements have been tabulated.