The preparation and characterisation of guanidines encapsulated in zeolite Y and anchored to MCM-41 are described. The catalytic activity of the immobilised and free guanidines were tested in the aldol reaction of benzaldehyde with acetone. In homogeneous phase, the guanidines give in 90-94% yield the condensation product alone. When anchored to MCM-41, the total yields range from 31% to 89% and the addition product is also formed depending on the solvent used. However, for the guanidine encapsulated in zeolite Y, the rate is strongly reduced and the addition product is preferentially formed in only 48% yield after 6 days. The reduced activity of this catalyst is explained by diffusional restrictions of the reactants and products inside the microporous system. (C) 1999 Elsevier Science B.V. All rights reserved.14841671173181