The far-infrared spectrum has been recorded Gum 50 to 360 cm(-1) at a resolution of 0.10 cm(-1) for normal propanal CH3CH2CHO and for the three isotopic species CH3CH2DO, CH3CD2CHO, and CD3CD2CHO. Along with the fundamental asymmetric torsional transition, several hot band transitions of the cis conformer have been observed and assigned for each isotopic species. The geometrical structures of the cis and gauche conformers have been determined by ab initio, calculations in order to compare them with previously reported microwave structures and to calculate inertial constants for the two internal rotations. A wide range of basis sets and ab initio theoretical methods up to MP4 have been used in order to determine the cia to gauche conformational energy difference. Using both the observed and calculated data, along with the previously reported gauche dihedral angle and microwave splittings for the gauche ground state for the CH3CH2CHO and CH3CH2CDO molecules, the asymmetric torsional potential function has been redetermined, from which the following constants have been obtained: V-1 = 125 +/- 15, V-2 = 365 +/- 17, V-3 = 456 +/- 20, and V-6 = -14 +/- 10 cm(-1). From this potential function, values of 787 +/- 20. 237 +/-12, and 442 +/- 20 cm(-1) were obtained for the cis to gauche, gauche to gauche, and gauche to cis barriers, respectively, while the enthalpy difference was constrained to 320 cm(-1). The fundamental and a few other excited-state methyl torsional transitions have been observed for both the cis and gauche conformers for all four isotopic species, and 3-fold barriers to internal rotation of 1084 +/- 40 cm(-1) (3.10 +/- 0.11 kcal/mol) for the cis-conformer and 790 +/- 56 cm(-1) (2.26 +/-0.16 kcal/mol) for the gauche conformer have been determined, The value for the cia conformer differs significantly from rile value reported rom microwave splitting data.