The existence of ether byproducts could be clearly proven for the hyperbranched polymerization of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) using NMR spectroscopy. Polymerization experiments were performed under standard conditions with different catalysts. The ether structures were identified by characteristic C-13 NMR signals of the quarternary carbons and of the ether methylene carbons. Different substitution pattern of these ether units could be distinguished. Using high-resolution magic-angle spinning (HR-MAS) NMR spectroscopy also cross-linked, swollen samples could be analyzed. The amount of ether side products was calculated from H-1 NMR spectra after alkaline hydrolysis of the polymer samples. Etherification can occur as an intramolecular reaction resulting in cycles. Intermolecular etherification results in molecules with two or more focal groups which can act as cross-linker in the polymerization. Thus, cross-linked samples were obtained at high degrees of conversion.