A miniaturization technique that allows the size of microstrip square open loop resonators to be reduced by more than 80% is presented and studied. The technique is based on the loading of the resonator with a series surface mount capacitor. It is shown that this technique allows the design of microwave bandpass filters with a wider stopband when compared with conventional designs. It is also proved that the insertion loss of the miniaturized filter is not degraded, but in fact can be maintained or even enhanced by the miniaturization process; this is true whenever the quality factor of the lumped capacitor is higher than the quality factor of the microstrip resonator. Finally, the feasibility of using the effect of the capacitor loss in the miniaturized resonator quality factor as a method to measure the effective series resistance of surface mount capacitors is studied, and recommendations towards its implementation are presented.