The invention relates to a process for the preparation of 5-formylvaleric acid or the corresponding 5-formylvalerate by hydroformylation of 3-pentenoic acid or 3-pentenoate ester with carbon monoxide and hydrogen in the presence of a catalyst comprising of rhodium and a phosphite ligand, wherein the phosphite ligand is represented by the following general formula ##STR1## where R.sup.1 and R.sup.2 are the same or different aromatic organic groups and where A is an n-valent group or atom and where n is an integer chosen upwards of 2 and where the respective �--O--P(OR.sup.1)(OR.sup.2)! group may be the same group or different groups and where the phosphite forms a chelate-type complex with rhodium.