Aldehydes can be converted to dienes by contacting the aldehydes with a non-zeolitic molecular sieve. Preferably the non-zeolitic molecular sieve has, in its calcined form, an adsorption of isobutane of at least about 2 percent by weight of the non-zeolitic molecular sieve at a partial pressure of 500 torr and a temperature of 20.degree. C. The non-zeolitic molecular sieves can achieve good conversion rates and selectivities for the reaction. The reaction is especially useful for the conversion of 2-methylbutyraldehyde to isoprene.