The synthesis of isoprene from the dehydration of 2-methylbutanal is described using aluminium phosphates as catalysts. Two samples of aluminium phosphate are studied prepared from the reaction of phosphoric acid with aluminium chloride or sulphate. The chloride route gives a mixed cristobalite/tridymite AlPO4 and this is shown to be more active than a catalyst containing only the tridymite form of AlPO4 formed from the sulphate route. The AlPO4 catalysts are also shown to be active catalysts for the synthesis of isoprene from 3-methylbutan-2-one, which is the major by-product formed from the reaction of 2-methylbutanal. The AlPO4 catalysts are deactivated due to the deposition of coke in addition to loss of phosphorus from the surface. Catalytic activity can be totally restored by a simple calcination procedure at 800(degrees)C.