At very high frequencies, the effect of losses in coupling devices makes itself felt in the determinations of the variousQ's and of the circuit efficiency of resonant systems. It is shown that the neglect of these losses (which appear in equivalent circuits in the form of series resistance), can lead to appreciable error. Formulas are derived and curves are plotted which permit an unambiguous determination of theQ's and of the circuit efficiency when the following items are known:1. the standing wave ratio at resonance,2. the standing wave ratio far from resonance, and3. knowledge as to whether the resonant system under study is ``under'' or ``overmatched'' to the external load.