The rate of the reversible aldol condensation of acetone (Ac) to diacetone alcohol (DAA) catalysed by Amberlite IRA-900 anion exchange resin in the hydroxide form is measured in the liquid phase, without solvent, using batch and continuous reactors. The desired reaction product is DAA, but its dehydration leads to the formation of mesityl oxide (MO) as a byproduct. Although the rate of DAA production is pore diffusion controlled, it may be modelled as second order with respect to [Ac] in the forward direction, and first order with respect to [DAA] in the reverse direction. Water in the reaction mixture plays a very important role in determining the reaction rate, the product selectivity, and the catalyst lifetime. Adding water to the reaction increases the catalyst’s lifetime and selectivity toward DAA, but slows down the rate of reaction.