Millimeter wave (MMW) imaging systems have the capability of detecting anomalous objects, which can include explosive threats and other prohibited items concealed on persons in airport checkpoints and other facilities requiring personnel screening. Benign materials that simulate explosives and other threats are used to test Advanced Imaging Technology (AIT) systems when live threats cannot be placed on human subjects. While laboratory dielectric measurements are used to formulate candidate simulants, it is useful, and sometimes necessary, to independently validate a simulant for an AIT system of interest. An imaging phantom has been fabricated using standard vacuum hardware and thin plastic films for containing samples of interest. The phantom’s design allows for simultaneous imaging of threats and candidate simulants in a fixed, repeatable fashion. The phantom was self-validated with deionized water using a criterion for resolving two overlapped distributions. Results obtained from a subsequent study of a flammable liquid versus its candidate simulant are presented, validating the use of the simulant for use with the target AIT system.