A process for preparing 2-methylbutyric acid having a reduced content of 3-methylbutyric acid from the secondary streams obtained in the preparation of pentanoic acids, includes generating a stream enriched with 2-methylbutanal and 3-methyl-butanal. The stream enriched with 2-methylbutanal and 3-methylbutanal of step is reacted with formaldehyde. The reaction with formaldehyde is followed by removal of the organic phase and selective hydrogenation in the presence of a hydrogenation catalyst with hydrogen at elevated temperature and elevated pressure and, after removal of the hydrogenation catalyst, treatment of the hydrogenation output obtained with an oxidizing agent.