The electron spin polarization and intensity of the specularly reflected () low energy electron diffraction (LEED) beam at an angle of incidence = 13° have been measured as a function of up to about 1 Langmuir of exposure to O2 and CO. Data have been acquired in the energy range 26-141 eV at 5 volt intervals except near sharp features in the polarization versus energy profile where smaller intervals were used. Data are presented in the form of polarization versus energy and intensity versus energy profiles for four selected exposures in the range measured. The sensitivity of polarization to exposure to the gases CO and 2 is comparable to the sensitivity of intensity. Polarization was found particularly sensitive to exposure in one small energy interval. The polarization as the intensity was found not to always monotonically decrease in magnitude with increasing exposure but may show more interesting behavior. A tendency for the complementary measurement to remain constant at energies where the other varies rapidly with exposure was observed. Differences for both polarization and intensity for the two gases are noticeable but not dramatic. The temperature dependence of electron spin polarization and intensity was also measured for this same clean surface over the range 1-3°C in the energy interval 5-11 ev, which includes the polarization versus energy profile. The effect of temperature on polarization was found to be greatest at energies for which the intensity is low and the polarization, and rate of change of polarization with energy, is large. The results may be Interpreted in terns of a shift of polarization features toward lower energies as a result of thermal expansion when the crystal is heated.