This paper presents a sensitive comparator/sensor useful to detect small differences between a reference sample and a sample under test (SUT). The device, a four-port structure, is based on a pair of identical microstrip lines each one loaded with an open split ring resonator (OSRR), the sensitive part of the device. The working principle of the comparator/sensor is mode conversion, caused when the OSRRs are asymmetrically loaded, and the output variable is the cross-mode transmission coefficient. The structure is very sensitive to changes in the complex dielectric constant between the reference sample and the SUT, and it is useful as comparator and sensor in applications such as defect detection, dielectric characterization of solids and liquids, and solute concentration measurements in liquids. The application examples reported in the paper confirm the high sensitivity of the structure to the complex dielectric constant of the SUT, caused by the specific shape of the OSRRs.