The vapor phase aldol condensation of formaldehyde with acetaldehyde has been investigated over montmorillonite-rich (Mt-rich) materials exchanged with Na+, Cs+, Mg2+, Ni2+, La3+ or UO22+ cations. The main products are acrolein (via cross-condensation), and crotonaldehyde (via self-condensation), indicating that aldol processes effectively take place over such catalysts. The results showed that ion-exchange induces different catalytic activities depending on the nature of the cation introduced and the reaction temperature. Both the acid and base properties exert strong effects upon the total conversion of acetaldehyde and the acrolein:crotonaldehyde ratio. It was also found that cross-condensation takes place exclusively over weakly acidic sites. The highest cross-selectivities were obtained over Na, Cs and Mg-containing catalysts.