We have examined the adsorption and reaction of CO with lanthana in order to understand the catalytic activity of lanthana for NO reduction by CO. In a series of experiments on films formed by vapor deposition of La2O3 in vacuum, it was shown that heating the lanthana in CO at 670 K resulted in carbonate formation, as evidenced by desorption of CO2 in TPD. When this reduced lanthana was reoxidized in O-18(2) and again heated in CO, significant quantities of labeled CO2 were observed in TPD. Heating the lanthana films in vacuum to 1400 K caused complete deactivation of the lanthana for carbonate formation with CO. Similar results were obtained on lanthana powders formed by nitrate decomposition. Again, heating the powders in CO resulted in significant carbonate formation and subsequent CO2 desorption in TPD-TGA measurements. Furthermore, calcination of the powders to 1700 K decreased the steady-state rates for NO reduction by CO by approximately two orders of magnitude, which was much larger than the decrease in surface area of the powders. The implications of these results for understanding the catalytic properties of lanthana are discussed.