Pancreatic colipase and its precursor, procolipase, facilitate interfacial lipid hydrolysis catalyzed by pancreatic lipase. To better understand how procolipase functions, its interactions with mixed-lipid monolayers at the argon-buffer interface have been characterized. The lipid mixtures consisted of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine and either 1,3-dioleoylglycerol, a model lipase substrate, or 13,16-cis,cis-docosadienoic acid, a model lipase product. Analysis of the lipid composition dependence of procolipase-induced surface pressure increases shows thermodynamically that procolipase interacts strongly and preferentially with the lipase substrate or product. This finding was confirmed by fluorescence measurements of procolipase interaction with pyrene lipid analogs. Analysis of the quantity of procolipase adsorbed to the lipid monolayers shows that interfacial packing obeys a simple, geometric model. The partial molecular areas obtained for procolipase (708 A2) and the phosphatidylcholine (70 A2) agree with their known cross-sectional areas. However, the areas for the fatty acid (14 A2) and diacylglycerol (18 A2) are less than half the expected values, indicating the formation of substrate multilayers. Overall, the results indicate a previously unrecognized role for procolipase, recruiting substrate laterally to its vicinity and, hence, to pancreatic lipase with which procolipase forms a 1:1 interfacial complex. Accompanying this preferential interaction of procolipase with lipase substrates is their rearrangement normal to the interface. These previously unrecognized properties of this lipase cofactor should have relevance for the regulation of other lipases, like lipoprotein lipase, which are regulated by cofactor proteins.