Thin films of Y‐Ba‐Cu‐O of different composition were grown on MgO and LaAlO3substrates by metal‐organic chemical‐vapor deposition. Using a microwave cavity resonator, their microwave surface resistance at 24.5 GHz was measured at a temperature of 77.5 K. It varies from less than 3 m&OHgr; to more than 50 m&OHgr; depending on the composition of the films. The lowest surface resistances could be obtained with samples having a composition close to the ideal stoichiometry 1:2:3 but with a slight excess of copper and yttrium. ©1995 American Institute of Physics.