A technique is described for the measurement of the refractive indexnof static or flowing lossy polar liquids in the 10‐GHz region and in the presence of magnetic fieldsH. Measurements ofnas a function ofHwere performed on static and flowing H2O containing various concentrations of ionic impurities. The application ofH(up to 7300 Oe) to liquids flowing with velocitiesv(up to 200 cm/sec) yielded the following result: &Dgr;n/n≤6×10−4for all concentrations. This result indicates that Umanskii's observed change in dielectric constant &egr;′ (roughly 100% change in &egr;′ forH= 1500 Oe andv= 50 cm/sec) is probably a spurious instrumental effect.