The surface area of platinum (as Pt black, and Pt dispersed on an inert support) may be obtained by measuring the uptake of hydrogen subsequent to chemisorption of oxygen by the metal. We have made a detailed study of this titration method at 196 K using evaporated platinum films, with particular attention to the accuracy and characteristics of the surface area obtained, and also to the effect of raising the temperature of the oxygenated surface, both in vacuo and in presence of gaseous oxygen. The magnitude of the surface area obtained refers only to the area catalytically active for the hydrogen + oxygen reaction at the temperature of determination, since not all the chemisorbed oxygen is removed by reaction with gaseous hydrogen at 195 K. Nevertheless, “active” area determinations have an accuracy of 3-4 %. The use of higher temperatures to effect desorption of the remaining “inactive” surface complexes must be exercised with caution since the amount of oxygen chemisorbed, well below 100°C, may exceed the monolayer.