The solid-acid (Bronsted)-catalyzed cyclo-dimerization of 3-hydroxy-3-methylbutan-2-one (HMB) over a synthetic ferrierite molecular sieve is reported. HMB is a stable liquid at ambient temperatures but in acidic solutions it readily undergoes reaction to generate a variety of products. However. in the acidic molecular sieve catalyst studied here, only one product - the cyclic dimer (proven by in situ solid state C-13 NMR and other evidence) - is observed, together with some unreacted HMB. A plausible, proton-catalyzed mechanism is proposed, and prompts comparison between the cyclo-dimerization of HMB within ferrierite and the mode of action of certain enzymes.