Electrically polarizable materials in an inhomogeneous electric field experience a dielectrophoretic force which is proportional to the gradient of the square of the electric field. For high frequency radiation, the time averaged electromagnetic field provides a dc force. The force exerted on a sapphire and a metal sphere by the microwave field in a single mode resonant cavity has been measured. The force measurements have been made at equally spaced points along three orthogonal axes centered in a cylindrical cavity operating in the TE111 mode using 20 watts of microwave power at about 4.84 GHz. These data are compared to the simple theory of dielectrophoresis on a small sphere and verify that, for this mode and in the absence of other forces, there is a unique and stable equilibrium position at the center of the cavity. Measurements of the quality factor of the cavity provide a means of making an absolute check to the theory. The TE111 mode is one of an infinite set of modes predicted to have stable equilibrium positions that exist away from the cavity walls. It is proposed that dielectrophoresis in a microwave resonant cavity can be used to position samples for material processing applications in a microgravity environment. Knowledge of these forces may also be important for interpreting thermogravimetric measurements made in a microwave oven.