Pentenenitrile oligomers formed in a process for isomerizing cis-2-pentenenitrile to 3-pentenenitrile are minimized in the presence of an aluminum oxide catalyst. The process comprises providing an aluminum oxide catalyst having an alkali metal and/or alkaline earth metal and/or iron content, measured in the form of alkali metal oxide and/or alkaline earth metal oxide and/or iron oxide, respectively of less than 5000 ppm by weight.