The invention relates to a method for the preparation of a polymerized CNSL composition, the method comprising subjecting a raw or technical CNSL to a heat treatment under reflux to obtain the polymerized CNSL composition, wherein the heat treatment is performed until the polymerized CNSL composition has a molecular weight Mw of about 2800 to about 3700, such as from about 2900 to about 3600, or from about 3000 to about 3500 g/mol. The invention further relates to a polymerized CNSL composition, use of a polymerized CNSL composition according to the invention in a bitumen composition, a modified bitumen composition according to the invention and an asphalt composition comprising a modified bitumen composition according to the present invention.