The structure of the weakly bound complex CO 2H 2CO has been determined by pulsed-jet Fourier-transform microwave spectroscopy. The spectroscopic constants of three isotopomers, CO 2H 2CO, 13CO 2H 2CO, and CO 2D 2CO, have been determined. Two distinct states of the complex are seen. These are caused by inversion doubling from "rotaion" of the formaldehyde about its twofold axis. The complex is planar with the C2 axis of formaldehyde almost parallel to the D∞ h axis of carbon dioxide. The center-of-mass to center-of-mass distance is 3.171 Å.