Microwave spectra of the trans–trans isomer of methylpropylether and its four deuterated species were measured. A plausible structure was estimated from the observed rotational constants. The dipole moment was determined to be 1.107 ± 0.005 D by Stark effect measurements. From the observed splittings of the spectra in the excited torsional state, the barrier to internal rotation of the CH3C group was obtained as 3280 ± 60 cal/mol.