The far-infrared and infrared spectra of gaseous dimethylallene (3-methyl-1,2-butadiene), (CH3)(2)CCCH2, were measured as well as the Raman spectra of the compound in all three physical phases. The spectra were assigned using infrared band contours, Raman depolarization ratios and ab initio predicted harmonic frequencies and intensities as criteria. The torsional energy level splittings were determined for the ground state and one torsional excited state from previously published microwave spectra by fitting the frequencies to an effective rotational Hamiltonian for molecules with two periodic large-amplitude motions. The splittings and the data from the far-infrared spectrum were used to derive the two-dimensional torsional potential function of dimethylallene. This function with an effective barrier V-3eff = 726(4) cm(-1) is compared with the results from the microwave analysis and with the potential function calculated by a number of ab initio methods (among them B3LYP/6-31G(d) and MP2/DZ(d)).