KF/gamma-alumina is an effective catalyst for the aldol condensation of citral with acetone. Even at a relatively low acetone/citral ratio of 1-10, the selectivity to pseudoionones was high, ranging between 82% and 97%. X-ray diffraction shows the presence of KF and K(3)AlF(6) crystallographic phases on the supported catalyst. At KF loadings of 3-6.75 mmol g(-1), a pretreatment of the catalyst by heating to 450 degrees C was necessary to obtain an active catalyst. However, samples with KF loadings of 8.5 mmol g(-1) and higher were active even without pretreatment, facilitating handling and use of the catalysts. The development of (1 1 1) planes revealed by in situ XRD during the thermal activation process correlates with the activity for aldol condensation. (C) 2010 Elsevier B. V. All rights reserved.