Process for preparing aldehyes

A process for preparing aldehydes comprising hydroformylating olefinically unsaturated compounds with hydrogen and carbon monoxide in a homogeneous phase in the presence of a catalyst system comprising rhodium complex compounds and aromatic phosphines in a molar excess and separating off the catalyst system from the hydroformylation reaction mixture by pressure filtration on a semipermeable membrane of an aromatic polyamide, the hydroformylation being effected at a pH of 2.5 to 4.3 using a molar ratio of phosphine:rhodium of at least 60 and at a rhodium concentration of at least 10 ppm by weight, based on the olefinically unsaturated compound used, and using, as aromatic phosphines, alklyammonium and/or arylammonium salts of sulfonated or carboxylated triarylphosphines of the formula ##STR3## wherein X is sulfonate (SO.sub.3.sup.-) or carboxylate (COO.sup.-), a, b and c are individually 0 or 1, wherein at least one of a, b, or c must be equal to 1, n is equal to 1, 2 or 3, R.sup.1 and R.sup.2 are individually selected from the group consisting of alkyl of 8 to 13 carbon atoms, C.sub.6 -C.sub.10 -aryl of 6 to 10 carbon atoms, cycloalkyl of 6 to 10 carbon atoms, and R.sup.1 can also be hydrogen, and the total of the carbon atoms in R.sup.1 and R.sup.2 must be at least 30.