Toxic amyloid aggregates are a feature of many neurodegenerative diseases. A number of biochemical and structural studies have demonstrated that not all amyloids of a given protein are equivalent but rather that an aggregating protein can form different amyloid structures or polymorphisms. Different polymorphisms can also induce different amounts of pathology and toxicity in cells and in mice, suggesting that the structural differences may play important roles in disease. However, the features that cause the formation of polymorphisms in vivo are still being uncovered. Posttranslational modifications on several amyloid forming proteins, including the Parkinson's disease causing protein α-synuclein, may be one such cause. Here, we explore whether ubiquitination can induce structural changes in α-synuclein aggregates in vitro. We used protein chemistry to first synthesize ubiquitinated analogues at three different positions using disulfide linkages. After aggregation, these linkages can be reversed, allowing us to make relative comparisons between the structures using a proteinase K assay. We find that, while ubiquitination at residue 6, 23, or 96 inhibits α-synuclein aggregation, only modification at residue 96 causes an alteration in the aggregate structure, providing further evidence that posttranslational modifications may be an important feature in amyloid polymorphism formation.