This invention is directed to a process for the conversion of an aldehyde to a diolefin comprising contacting an aldehyde of 4 to 6 carbon atoms in the vapor phase at a temperature of 300.degree. to 400.degree. C. with a catalyst comprising boron phosphate wherein the initial molar ratio of phosphorous (P) to boron (B) (P/B) is less than 1.0 but greater than 0.6 which is in intimate admixture with from 0.1 to 10 weight percent graphite. More specifically, this invention is concerned with a process for the catalytic dehydration of 2-methylbutanal (2MBA) to isoprene in the vapor phase with a boron phosphate catalyst wherein the improvement comprises the use of a catalyst with a molar ratio of P/B of less than 1.0 but greater than 0.6 which contains from 0.1 to 10 weight percent graphite and 0 to 10 mole percent, based on moles of boron, of ammonia or an amine selected from the group consisting of mono-, di- and/or tri- alkyl amines of 1 to 10 carbon atoms.