Radiofrequency and microwave rotational spectra of the complexes HCN CO2 and DCN CO2 have been obtained using molecular beam electric resonance spectroscopy. The spectra are characteristic of prolate asymmetric rotors. The spectroscopic constants obtained are HCNCO2 DCNCO2 [A-(B+C/2)](MHz) 9817(86) 9750(650) (B+C/2)(MHz) 2044.056(10) 1929.53(24) (B-C/2)(MHz) 182.42(60) 162(4) ΔJ (kHz) 0. 0070(6) δK (kHz) 137(50) 229(32) μa (D) 3.2067(32) 3.2279(29) eqQaN (MHz) -4.075(14) ... These are shown to be consistent with a C2ν structure in which the nitrogen of the HCN bonds to the carbon of the CO2 with a weak bond length of 3.00 Å. The HCN subunit subtends an average angle of 17.4(2)° with the line joining the centers of mass of the two submolecules, and the average bending vibrational amplitude of the CO2 is 11(1)°. The difference between the average amplitudes of the in-plane and out-of-plane bending vibrations of the HCN is less than 4°. The stretching force constant for the weak bond is 0.049 mdyn/Å in HCNCO2, and the induced dipole moment is 0.361 D.