Infrared spectra of n-propyl chloride and bromide have been measured in the gaseous, liquid and solid states. The Raman measurement was also made in the liquid state. From the normal vibration calculation and by comparison with the spectra of ethylene chlorhydrin it has been concluded that in the liquid state both the trans and gauche forms are present and of these only the trans form persists in the solid state. The energy difference between the trans and the gauche forms has been determined by the absorption intensity measurement.