The heterogeneous gas-phase dehydration of the tertiary alcohol, 1-methylcyclohexanol, and the secondary alcohol, cyclohexanol, over metal(IV) phosphate catalysts proceed at approximately equal rates. Under these conditions, cyclohexanol is known to react through a carbocationic mechanism. However, through use of deuterium-labelled substrates, it has now been shown that some of the major product (1-methylcyclohexene) from dehydration of 1-methylcyclohexanol arises by loss of hydrogen from the methyl group by a synchronous mechanism. The labelling results are consistent with either a synchronous or, more likely, a mixed synchronous/carbocationic mechanism.