Microwave spectra of CH2CO-C2H4, CD2CO-C2H4, CH2CO-CH2=CD2, CH2CO-cis-CHD=CHD, and CH2CO-trans-CHD=CHD have been measured with a pulsed-beam Fourier transform microwave spectrometer. A b-type spectrum is observed with transitions split into as many as four states due to tunneling motions which exchange the ketene hydrogens and the two pairs of hydrogens oriented trans to one another in ethylene. The components of the electric dipole moment along the a and b principal axes were determined to be mu(a) = 0.160(30) x 10(-30) C m [0.048(9) D] and mu(b) = 4.5875(33) X 10(-30) 0 m [1.3753(10)D]. A structure having C-s symmetry is found with ketene lying in the symmetry plane of the complex and the molecular plane of ethylene crossing the symmetry plane at 90 degrees with a carbonyl carbon-ethylene center of mass distance, R, of 3.460(2) Angstrom. Microwave and electrostatic modeling calculations show that the angle between the b-axis of ethylene and R is 49(3)degrees. The distributed multipole model also accounts for the planar geometry of the ketene-acetylene complex as compared to the crossed structure of the ketene-ethylene complex in terms of differences in the quadrupole moments of the two hydrocarbons. The ketene-ethylene complex has a structure which is qualitatively similar to theoretical determinations of transition-state geometries of the CH2CO + C2H4 cycloaddition reaction. The uncertainties shown here and throughout this paper are la, the standard deviation determined by least-squares fitting.