The radio frequency and microwave spectra of NH3-CO2 have been measured using the molecular beam electric resonance technique. The spectrum is characteristic of an asymmetric top in which the NH3 subunit exhibits effectively free internal rotation. The spectroscopic constants obtained for the ground internal rotor state are presented below: B+C/2(MHz) 3756.178(3), B-C/2(MHz) 597.4(2), A-B+C/2(MHz) 8035(8), ΔJ(MHz) 0.0240(4), δK(MHz) 0.20(2), ΔJK(MHz) 0.23(5), δJ(MHz) 0.007(1), eQqaa(MHz) -3.175(4), eQq bb(MHz) 1.557(9), eQqcc(MHz) 1.617(11), μ(D) 1.7684(14). The N-CO2 framework of the complex has C2v symmetry with a N-C weak bond length of 2.9875(4) Å. The average bending angle of the NH3 subunit is 22.71(5)° with a difference in amplitude of 1.0(4)° between the in plane and out of plane excursions. The weak bond stretching force constant is 0.070(1) mdyn/Å and the induced dipole moment is 0.411(2)D.(B+C)/2 for the first excited internal rotor state (|m|=1) is 3753.008(4) MHz and the quadrupole coupling constant eQq aa |m|=1=-3.176(9) MHz is identical with that measured for the ground internal rotor state.