The present invention relates to a process for producing a polycondensation product by polycondensing monomers including (A) an aromatic or heteroaromatic compound containing a polyether chain, (B) optionally an aromatic or heteroaromatic compound which is different from monomer (A), and (C) an aldehyde, in the presence of a protonation catalyst, wherein the aldehyde is provided by a fast-releasing aldehyde source and a slow-releasing aldehyde source.
