The electroluminescence and photoluminescence light‐emitting devices based on poly(p‐phenylene vinylene) (PPV), with emitting layers of different thicknesses, were examined. It was shown that microcavity effects occur in thin PPV single‐layer sandwich structures (ITO/PPV/Al, Au/PPV/Al) in both electroluminescent and photoluminescent emission. Microcavity effects were demonstrated for a range of different PPV layer thicknesses down to 40 nm. We have successfully prepared resonant‐cavity single‐layer light‐emitting diodes with enhanced external quantum efficiencies of up to 0.1% photons/electron, with aluminum as the electron injecting and gold as the hole injecting electrode. Less pronounced microcavity effects were observed on devices with ITO as the hole injecting contact, due to the low reflectivity of the ITO‐glass interface. These findings were confirmed by theoretical simulations based on an optical transfer matrix formalism. ©1996 American Institute of Physics.