INTRODUCTION Plastic foam is being used successfully in some fixed roof tanks to reduce the evaporation loss from crude oil. Plastic foam consists of nitrogen-filled@ spherical. hollow particles made from several types of resin. Diameters are in the range of 2 to 150 microns (nominal diameter@ 0.001 in.). Floating upon oil@ the foam forms a barrier between the liquid and vapor phase; it retards evaporation in much the same way as a floating roof does. The foam flows around roof supports and re-forms into a continuous@ flexible blanket which makes it practical for use in tanks having roof supports. The effectiveness of plastic foam in reducing evaporation loss from crude oil and gasoline has been tested by industry. Many of these tests were conducted because of encouragement by the Evaporation Loss Committee of the American Petroleum Institute. This bulletin fulfills the committee's commitment to provide an exchange of the information developed. It is limited to crude oil because tests have shown that foam is less effective and causes greater operating problems when used for gasoline service. Proper appraisal of a plastic-foam installation requires knowledge of the following items; 1. the volume of hydrocarbons saved; 2@ the practical unit value of the hydrocarbons saved (the components may have higher or lower unit value than crude oil); 3. the replacement cost incurred to balance the foam-particle disappearance rate; and@ 4@ the debit for any long-term sacrifice in tank capacity or in flexibility of frequency of use. Items 2 and 4 are sometimes overlooked. This bulletin provides knowledge which enables a potential user to estimate whether plastic foam can be of economic benefit to him. Test data are interpreted. Anticipated loss-reduction factors are established for static and working services. Variables are discussed which appear to govern the disappearance rate of the foam from the useful blanket form. Practices are suggested for initial operation to prolong the useful life of the foam until local test work can establish acceptable degrees of relaxation of these practices. Also@ other effects??reduction of corrosion and of hydrogen sulfide (H2S) concentration??are presented.