A catalyst containing at least one amidine or guanidine group of formula (Ia) or (Ib), which is bound to a silicon atom of a siloxane residue. At room temperature, the catalyst is liquid and odorless. It is particularly suitable as a cross-linking catalyst for curable compositions, in particular for silane group-containing compositions. It is particularly good at accelerating the hardening of such compositions without impairing stability in storage, and displays little volatility but good compatibility. As a result, the compositions do not tend towards separation or migration or evaporation of the catalyst.