A process for preparing SiC-bonded polyethersiloxanes by precious metal-catalysed addition of olefinically unsaturated polyethers onto siloxanes having SiH groups is described, wherein, prior to commencement of reaction, the siloxane having SiH groups is mixed with an olefinically unsaturated polyether and with a polyethersiloxane by stirring, and then contacted with a precious metal catalyst and optionally further additives, and the SiC bond formation reaction is conducted up to an SiH conversion, determined by gas-volumetric means, of at least 97%.