Pulsed-beam Fourier transform microwave spectroscopy was used to study ethanethial S-oxide, an unstable species generated in a (Z)/(E) ratio of 97/3 by pyrolysis of 2-methyl-2-propylvinyl sulfoxide at 350 degrees C in Ar or Ne/He gas flows. Rotational transitions of the (Z) normal isotopomer exhibited A-E state splittings due to internal rotation of the methyl group. A global fit of the observed A and E state lines of the (Z) isomer io an internal rotor Hamiltonian gave rotational constants and centrifugal distortion constants of A = 14237.0861 (5) MHz, B = 5678.4488 (3) MHz, C = 3594.8008 (2) MHz, Delta(JK) = -9.43 (55) kHz, Delta J = 3.48 (11) kHz, and delta(J) = 1.11 (2) kHz and a 3-fold internal rotor barrier of 285.6 (3) cm(-1). No internal rotation splitting was observed for the (E) isomer and a pseudorigid rotor fit of the observed transitions of the normal isotopomer gave A = 31 128 (27) MHz, B = 3475.8521 (16) MHz, C = 3188.4429 (23) MHz, and Delta(J) = 0.49 (12) kHz, Microwave spectra of eight (Z) isotopomers were assigned, and a partial substitution structure was derived : r(C=S) = 1.618 (3) Angstrom, r(S-O) = 1.477 (4) Angstrom, r(C-C) = 1.493 (3) Angstrom, theta(CSO) = 113.9 (2)degrees, and theta(CCS) = 125.4 (2)degrees. The electric dipole moment components of the (Z) isomer along the a and b principal axes were measured to be mu(a) = 2.714 (5) D and mu(b) = 1.869 (35) D, respectively.