The present invention relates to a novel process for the preparation of a pyrazole carboxylic acid derivative of the formula wherein R1 is C1-7-alkyl and R3 is C1-7-alkyl which is optionally substituted with halogen or C1-4-alkoxy. The pyrazole carboxylic acid derivative of the formula I can be used as building block in the preparation of pharmaceutically active principles e.g. for compounds acting as phosphodiesterase (PDE) inhibitors, particularly PDE10 inhibitors. PDE10 inhibitors have the potential to treat psychotic disorders like schizophrenia.