Mid-filmr is a process which has been developed to combine the high resolution capability of thin film technology and the nonvacuum low cost of thick film technology. In this paper the performance of mid-film microstrip transmission lines is described for applications in microwave integrated circuits (MIC's) in the frequency range of 3-12 GHz. The experiments were carried out on a number of ring resonators with characteristic impedances of 35, 50, and 71Omeg. The resonators were fabricated using mid-film gold conductors on 96 percent and 99.6 percent alumina substrates. Theoretical calculations of microstrip losses were carried out and compared with microstrip attenuation measurements using a swept frequency technique. The results showed that the attenuation in midfilm microstrip transmission lines is esentially identical to the attenuation in thick film lines using the same conductor materials with the added advantages of fine line resolution capability (50-µm line widths and spaces) and lower cost of mid-films. Typically, the attenuation constant of 50Omeglines on 99.6 percent alumina was 0.06 dB/cm at 7 GHz while the theoretical lower bound is 0.05 dB/cm. In addition a Wilkinson 3-dB in-phase power divider has been fabricated using mid-film gold microstrip branches which were isolated by a 100-Omegthick film resistor (EMCA5512). The mid-film Conductor was found to be compatible with the thick film resistor and the Wilkinson 3-dB hybrid behaved as expected with a loss of 0.3 dB and isolation of greater than 30 dB over a 500-MHz band centered at 6 GHz.