Alkoxy RTV-1 compositions comprise the product of the reaction of(A) an HO-terminated organopolysiloxane,(B1) an alkoxysilane which has at least three alkoxy groups and/or its partial hydrolysates, and(B2) an alkoxysilane which has two alkoxy groups and/or its partial hydrolysates, in the presence of(C) an acid phosphoric ester of the general formula (I)wherea is 1 or 2,R1 and R2 are a hydrogen radical, methyl radical or hydroxyl radical,b and d are 2 or 3,c is an integer from 2 to 15,e is 0 or 1L comprises a radical -O-, -COO-, -OOC-, -CONR3-, -NR4CO- or -CO-,R3 and R4 are a hydrogen radical or C1-C10-alkyl radical, andM is a monovalent, unsubstituted or hydroxyl-, fluorine-, chlorine-, bromine-,C1-C10-alkoxyalkyl- or cyano-substituted C1-C20-hydrocarbon radical, with the proviso that on any given carbon atom not more than one radical R1 and R2 may be a hydroxyl radical.