The cleavage energies of single crystals of NaCl and MgO were measured in vacuum and in liquid nitrogen. The average value of the cleavage energy of NaCl while submerged in liquid nitrogen was 317 erg/cm2as opposed to 283 erg/cm2in vacuum at 77°K. The cleavage energy of MgO is 1300 erg/cm2, which agrees with values obtained at 300°K in air and in liquid nitrogen. This lack of atmospheric influence may be due to the inability of contaminants to diffuse into the crack tip of the specimen.