In this paper, a low-loss balanced microstrip common-mode (CM) isolator based on slot lines is proposed. Two microstrip-driven signals in opposite phase are combined into a slot line with a special geometry. Placed back-to-back, this configuration provides a good balanced-microstrip to balanced-microstrip common mode isolation. A transmission line CM model is proposed. Measurement results show −22dB of commonmode rejection as well as a differential mode (DM) insertion loss as low as −0.15dB at 2.45 GHz. This low loss structure is of particular interest for balanced feed structures using two out of phase unbalanced amplifiers.