The synthesis of isoprene from the dehydration of 2-methylbutanal is described using boron phosphate, aluminium phosphate, and mixed boron/aluminium phosphates as catalysts, Both boron phosphate and aluminium phosphate deactivate steadily with reaction time due to loss of catalyst activity but the selectivity to isoprene is not significantly affected by catalyst deactivation. Catalyst deactivation is shown to be due to two factors: (i) loss of surface phosphorus and (ii) coke formation. Reactivation of the catalysts at temperatures up to 500 degrees C in an air atmosphere does not successfully restore the catalyst activity, although this procedure does remove all the coke. It is shown that high-temperature calcination (800 degrees C) removes both the surface carbon and restores the surface phosphorus content, and hence this procedure is a necessary prerequisite for the successful reactivation of boron and aluminium phosphate as a catalyst for 2-methylbutanal dehydration. Two samples of aluminium phosphate were studied, prepared from the reaction of phosphoric acid with aluminium chloride or sulfate. The chloride route gives a mixed cristabolite/tridymite AlPO4 and this is shown to be more active than a catalyst containing only the tridymite form of AlPO4 formed from the sulfate route. However, both are less active than BPO4 which can be readily prepared in the cristabolite structure, Mixed B/AlPO4 catalysts (Al:B mol ratio = 1:0.05 and 1:0.1)have also been investigated and these are shown to have a superior catalytic performance when compared with undoped AlPO4 (31)p, Al-27, and B-11 MAS NMR spectroscopy shows that B and Al are in the same lattice in these mixed phosphate catalysts. Addition of Nb is shown to stabilize the catalytic performance. The BPO4 and AlPO4 catalysts are also shown to be active catalysts for the synthesis of isoprene from methyl isopropyl ketone, which is the major by-product formed from the reaction of 2-methylbutanal. It is suggested that a process for the synthesis of isoprene based on the dehydration of 2-methylbutanal would involve the recycle and conversion of the by-products. The mechanism of the reaction is discussed and it is proposed that 2-methyl-but-2-en-1-ol is an intermediate central to the formation of the two major products of the dehydration reaction: isoprene and methyl isopropyl ketone.