[en] Results are presented on the difference of R, the ratio of longitudinally to transversely polarized virtual photon absorption cross sections, on deuterium and hydrogen targets. They were obtained by comparing the ratio of cross sections for the deep inelastic scattering of muons from deuterium and hydrogen at 90 and 280 GeV incident energy. The results cover the range x=0.01-0.30, at a typical Q² of 10 GeV². The measured difference R^d-R^p shows no significant x-dependence and is compatible with zero, as well as with expectations from perturbative QCD. (author) 7 refs.; 2 figs