Deuterium isotope effects on the methyl group conformation and ESR spectra of selectively deuterated dimethyl ether radical cations are demonstrated by employing experimental matrix-isolation ESR techniques in combination with accurate ab initio and density functional theory (DFT) quantum chemical methods. The experimental study shows strong deuterium isotope effects on the proton hyperfine coupling constants (HFCC) for CD3OCH3+, CD3OCH2D+, and CD3OCHD2+ as well as a temperature dependence in the HFCC for CD3OCH2D+ and CD3OCHD2+. The deuterium isotope effects and temperature dependencies can be understood by incorporating the mass difference of the two hydrogen isotopes in addition to their magnetic properties, and utilizing new, improved quantum chemical structure calculations. Good agreement between experiment and theory is obtained when correlated ab initio (MP2) geometries are used in conjunction with DFT hyperfine coupling constants, whereas an erroneous temperature behavior is obtained for the HFCC when older methods or DFT calculations are used for the geometry determinations.