Oligomeric and protofibrillar aggregates that are populated along the pathway of amyloid fibril formation appear generally to be more toxic than the mature fibrillar state. In particular, α-synuclein, the protein associated with Parkinson's disease, forms kinetically trapped protofibrils in the presence of lipid vesicles. Here, we show that lipid-induced α-synuclein protofibrils can convert rapidly to mature fibrils at higher temperatures. Furthermore, we find that β-synuclein, generally considered less aggregation prone than α-synuclein, forms protofibrils at higher temperatures. These findings highlight the importance of energy barriers in controlling the de novo formation and conversion of amyloid fibrils.