This study introduces a new approach for producing highly monosized polymer microcapsules. The technique employs an impressive process in which the solute, dissolved freely in a solvent, is diffused uniformly into the preexisting monosized polymer particles in the form of a fine emulsion. The solvent is then eliminated, resulting consequently in solute-loaded polymer microcapsules. We named it a solute co-diffusion method, SCM. The loading efficiency of the solute was predicted successfully by a thermodynamic equation. In the experiment, the indomethacin-loaded poly(methyl methacrylate) microparticles were produced via SCM. From our results, SCM is expected to expand significantly the drug-loading technology in related fields.