Pulsed-beam Fourier transform microwave spectroscopy was used to study propanethial S-oxide (1), an unstable species generated in a (Z)/(E) ratio of 98/2 by pyrolysis of 2-methyl-2-propyl-1'-propenyl vinyl sulfoxide at 350 degrees C in Ar or Ne/He gas flows. A fit of 25 transition frequencies to a Watson "A" reduced Hamiltonian gave the following rotational constants and centrifugal distortion constants for the normal isotopomer of (Z)-1a: A = 10 182.2558(3) MHz, B = 2209.5000(9) MHz, C = 1997.1734(8) MHz, Delta(JK) = -62.69(2) kHz, Delta(J) = 4.165(3) kHz, delta(J) = 0.3336(3) kHz and delta(K) = -31.7(4) kHz. Twelve transitions were fit in the same manner for (E)-1b to give A = 16231(86) MHz, B = 1823.6154(7) MHz, C = 1785.7215(7) MHz, Delta(JK) = -12.5(5) kHz, Delta(J) = 0.48(1) kHz, delta(J) = -0.060(8) kHz. Microwave spectra of six (Z) isotopomers were assigned and a partial substitution structure was derived: r(C=S) = 1.585(6) Angstrom, r(S-O) = 1.473(2) Angstrom, r(=CH-CH2) = 1.513(7) Angstrom, r(CH3-CH2) = 1.536(3) Angstrom, theta(CSO) = 113.8(2)degrees, theta(CCS) = 126.7(3)degrees, and phi(CCCS) = 118.4 degrees. The electric dipole moment components of the (Z) isomer along the a, b, and c principal axes were measured to be mu(a) = 2.59(2) D, mu(b) = 2.11(2) D, and mu(c) = 0.33(6) D, respectively, which gives a total electric dipole moment of 3.35(2) D.