The adsorption of oxygen, hydrogen and carbon monoxide has been studied on evaporated films of nickel prepared under varied conditions with a view to defining more precisely the differences in behaviour between nickel films and nickel powder. Heats of adsorption on the films have been determined at room temperature using a resistance-thermometer calorimeter. The extent of the adsorption with oxygen and hydrogen has been assessed by means of krypton isotherms at 77°K, and by means of established correlations between activity and weight for equivalent films. The coverage with oxygen reaches a complete monolayer, but with hydrogen at 0·1 mm pressure, the coverage is estimated at 40%. Curves of differential heat versus coverage have been approximated by making heat measurements for small increments. With oxygen, a study over 15 increments shows a heat of adsorption which is constant near 150 kcal/mole for 80% of the uptake, but which falls ultimately to a value well below 50 kcal/mole. With hydrogen, there is a continuous, almost linear fall with coverage from an initial heat near 40 kcal/mole to a final heat near 10 kcal/mole. The reversibility of the hydrogen adsorption has been specifically examined. The films as originally prepared are porous with surface areas of 30 m
2
/g or higher, but the act of chemisorption appears to initiate sintering. The integral heats with each gas studied are, without exception, greater than those reported for nickel powders. From the collated results, it is suggested that the enhancement of the heat is related to the degree of disorder of the metal specimen, and attention is drawn to the possibility of release of stored energy from films on rapid chemisorption of gases. Heats of adsorption have also been measured on 'oxided' films obtained by heat treatment
in vacuo
of oxygenated surfaces. It is apparent that the behaviour of the nickel films approaches that of powders at high coverages of chemisorbed gases, or, equally, after sintering.