The microwave spectra of morpholine and N-deuteromorpholine have been investigated within the 8 to 40 GHz region. For both the normal and the deuterated species, strong a-type spectra which are due to the chair conformer with an equatorial imino group were recorded. The ground vibrational state rotational constants and quadrupole coupling constants are (in MHz), for normal morpholine, A = 4924.88 ± 0.05, B = 4625.15 ± 0.05, C = 2684.25 ± 0.05, χaa = 2.22 ± 0.02, χbb = 2.51 ± 0.04, χcc = −4.73 ± 0.02, and for N-deuteromorpholine, A = 4925.39 ± 0.05, B = 4399.67 ± 0.05, C = 2607.09 ± 0.05, χaa = 1.92 ± 0.02, χbb = 2.59 ± 0.04, χcc = −4.51 ± 0.02. The rs coordinates of the imino hydrogen are (in Å) aH = 2.373 ± 0.005 and cH = 0.10 ± 0.03. For the normal species the dipole moment components are (in D) μa = 1.68 ± 0.01, μc = 0.30 ± 0.01, μ = 1.71 ± 0.02. The value of aH and the direction of μ with respect to the a and c principal axes confirm that the observed spectrum is that of the chair equatorial conformer of morpholine.