a process is disclosed for producing a 3 - pentenoic ester which comprises reacting butadiene , carbon monoxide and an alcohol in the presence of cobalt carbonyl catalyst , characterized in that the reaction is carried out in a reaction medium composed of at least two solvents selected from the group consisting of pyridine , quinoline , isoquinoline and substituted pyridine , substituted quinoline and substituted isoquinoline in which substituent or substituents are selected from the group consisting of alkyl having 1 - 6 carbon atoms , alkenyl having 1 - 6 carbon atoms , aryl , alkylaryl having 7 - 10 carbon atoms and aralkyl having 7 - 10 carbon atoms .