Transition moment directions of mid-infrared transitions in four isotopomers of propene, CH2=CHCH3, CH2=CDCH3, CD2=CHCD3, and CD2=CDCD3, have been determined from linear dichroism spectra recorded in stretched polyethylene as solvent. The results were compared with expectations based on bond directions and with results of ab initio calculations at levels of approximations ranging up to CCSD/6-311G**. The former procedure is found to be unreliable, even for high frequency vibrations. With rare exceptions, the highest levels of ab initio theory agree with the experimental directions to within about +/-20 degrees, and for intense transitions, to within the experimental uncertainty of about +/-(5-10)degrees.