This paper explores the possible effect of the potassium precursor on the activity of K/MgO catalysts for soot combustion and, in particular, which of previously described potassium roles would be more affected. The catalytic activity of the prepared K-catalysts was evaluated through thermogravimetric assays using catalyst-carbon black mixtures. Selected catalyst samples were characterized by XPS, AAS, BET surface area, carbothermic reduction, DRIFTS and XRD. The effect of physical contact was also assessed. The experimental results show that the addition of potassium from different precursors (KOH, KNO3) affects the basic nature of the catalyst surface and its interaction with active species, and this is reflected in the catalytic activity. With KNO3 as the precursor, a more active catalyst was obtained, a result that is attributed to: (i) higher concentration and dispersion of potassium, (ii) greater mobility and reactivity of the activated oxygen species, and (iii) lower stability of surface carbonates and OH groups, facilitating the formation of active species and favoring the mobility of surface activated oxygen. (c) 2006 Elsevier B.V. All rights reserved.