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With the aim of astronomical observation, the millimeter-wave spectrum of {\it iso}-propanol has been observed in the 100-370 GHz region with the FASSST spectrometer. Over 11000 transitions within three torsional substates and between two {\it gauche} substates have been successfully assigned up to $J″=68$ with the CAAARS program. The rotation of the hydroxyl portion gives {\it iso}-propanol three stable configurations labeled {\it trans}, {\it gauche} and {\it gauche'} corresponding to the dihedral angle of H-C-O-H. The two equivalent {\it gauche} forms interact through tunneling to split the degeneracy in energy. The resulting torsional substates can be characterized by the labels {\it gs} (symmetric {\it gauche}) and {\it ga} (anti-symmetric {\it gauche}). Previous microwave and infrared spectroscopic studies concluded that {\it gs} is the global minimum and {\it trans} is the highest in energy. In our new simultaneous fit of {\it gs} and {\it ga} data using a two-state Hamiltonian, it was necessary to include up to fifth-order torsion-rotation interaction terms as well as sextic centrifugal distortion terms to analyze the millimeter-wave spectrum of {\it iso}-propanol. The lease-square fit of {\it gauche} resulted in a RMS of 88 kHz. The {\it trans} substate was analyzed with a rotational Hamiltonian for a quasi-rigid molecule with up to sextic centrifugal distortion terms with a RMS of the fit 63 kHz. Despite a number of theoretical studies as well as experimental work, the torsional energy of the {\it trans} conformer has not yet been determined because of a lack of direct measurements of torsional transitions between the {\it trans} and {\it gauche} substates. We estimated the {\it trans-gauche} energy separation from relative intensity data.