The covalent attachment of polyethylene glycol (PEG) to a therapeutic protein is frequently used to increase the half-life of that protein in patients while reducing their immunogenic response. The PEGylation reaction can be optimized to maximize the desired product (average moles of PEG per mole of protein) molecules while limiting undesired byproducts (under- and over-PEGylated species). In the example discussed here, the desired molecule was mono-PEGylated, yet the preoptimized reaction yielded multi-PEGylated species visible by SDS-PAGE. The goal of our experiments was to maximize the amount of mono-PEGylated product produced during the PEGylation reaction while minimizing the costs associated with manufacturing the mono-PEGylated product.