This paper describes a method to concentrate solutes and colloidal entities, from small ions and molecules to proteins and nanoparticles, within individual aqueous microdroplets in oil. The mechanism lies in the entrapment of the solutes within an aqueous microdroplet, while the water molecules from the droplet slowly dissolve into the organic phase. Because the rate of change in concentration scales as the fifth power of the surface-area-to-volume ratio of the droplet, this phenomenon is prominent mostly in the micrometer-length scale. This paper presents measurements that quantify the degree of solute entrapment within the microdroplet and further describes the dynamics of droplet shrinkage and the factors that influence the rate of shrinkage. In addition, this paper explains why this concentration effect does not occur for certain organic microdroplets in aqueous solutions.