An inversion of solar active region microwave polarization was observed during the Coronal Magnetic Structures Observing Campaign (CoMStOC) [Brosius et al., 1992; Nitta, 1991; Schmelz and Holman, 1991; Schmelz, 1991; Schmelz et al., 1988]. This campaign, carried out in November and December 1987, was intended to improve our understanding of active region coronal plasmas and magnetic fields. The intensity and polarization of solar active region microwave emission provides probes of the coronal magnetic field. When combined with simultaneous x-ray and photospheric magnetic field observations, they form a powerful data set that can be used to deduce the physical properties of the plasma and the magnetic field in the active region corona. This article describes some results from a highly successful campaign.