Many notable discoveries have resulted from the characterization or purification of compounds by electrophoresis. The results reported here show that when polyacrylamide gels are used as the support matrix for electrophoresis, proteins can be modified by reaction with unpolymerized monomers of acrylamide, forming covalent acrylamide adduction products. This was demonstrated by electrophoresis of human and rat hemoglobins and subsequent analysis by electrospray ionization mass spectrometry. The degree of covalent acrylamide binding can be appreciable. The degree of adduction is dependent upon the condition of the gel matrix, the amount of material applied, and the reactivity of the material under investigation.