A laser ablation device in combination with a molecular beam Fourier-transform microwave spectrometer has allowed the observation of the rotational spectrum of solid thiourea for the first time. The sensitivity reached in the experiment allowed the observation of the isotopomers S-34, C-13, and N-15 in their natural abundance. The spectrum of D-4-thiourea was also analyzed from an enriched sample. The complicated hyperfine structure arising from the presence of two N-14 quadrupolar nuclei has been fully resolved and analyzed. The substitution r(s) structure has been derived from the experimental moments of inertia. Thiourea in gas phase presents a planar heavy atom skeleton. Experimental inertial defect values and high-level ab initio calculations reveal that the amino groups hydrogen atoms lie out-of-plane with a C-2 symmetry configuration and are involved in large amplitude inversion motions. (C) 2004 American Institute of Physics.