We report measurements of the magnitude and phase of microwave transmission through thin films (1 m) of the 90-K superconductor YBa2Cu3O7 on Al2O3, MgO, and ZrO2 substrates. Real and imaginary parts of the conductivity, 1 and 2, were evaluated from the measurements by comparing with a calculation of the transmission for our experimental configuration. Both parts of the conductivity show qualitative agreement with Bardeen-Cooper-Schrieffer type conductivities. From 1 we find a range of gap values 2kBTc=1.3 to 6.5. From the measured 1 the magnetic penetration depth can be evaluated. © 1988 The American Physical Society.