An alkane-like contaminant was isolated from as-received commercial PMMA by forming a conventional urea inclusion compound during the preparation of a PMMA-urea complex. The contaminant, likely a surfactant used in the emulsion polymerization of PMMA, acts as a plasticizer in the commercial PMMA sample. After removing the contaminant, the resultant coalesced-PMMA showed an approximately 30 °C higher glass transition temperature than the as-received PMMA. Forming the PMMA-urea complex and then removing urea and the contaminant together with methanol can purify commercial PMMA and “unmask” its true Tg. Considering the large amount of research performed on PMMA blends, thin films, and nanocomposites, often with inconsistent or conflicting results, we believe the purity of some PMMAs used in past and future studies needs to be examined. Because commercial polystyrene is also obtained by emulsion polymerization and its blends, thin films, and nanocomposites have also been extensively studied, a similar contaminant may be affecting its behavior.