A process for preparing compounds of the formula IIb  where R is C6-C30-alkyl or C6-C30-alkenyl, which may additionally have upto 5 hydroxyl groups, formyl groups, C1-C4-alkoxy groups, phenoxy groups or C1-C4-alkoxycarbonyl groups as substituents and may be interrupted by upto 5 nonadjacent oxygen atoms, or alkoxylate groups of the formula —(CH2)k—O—(A1O)m—(A2O)n—Y where A1 and A2 are, independently of one another, 1,2-alkylene groups having 2 to 4 carbon atoms, Y is hydrogen, C1-C12-alkyl, phenyl or C1-C4-alkoxycarbonyl, and k is 1, 2 or 3, and m and n are each numbers from 0 to 50, and the total of m+n must be at least 4, by reacting iminodiacetonitrile with aldehydes of the formula R—CHO and HCN or alkali metal cyanides, the process being carried out a) in the absence of an organic solvent and in the presence of a Lewis or Brönsted acid, or b) in the presence or absence of an organic solvent and in the presence of an emulsifier, or c) in the presence or absence of an organic solvent and under a pressure in the range from 1 to 40 bar.