Abstract: The invention relates to a process for the dimerization of derivatives of acrylic acid in the homogeneous phase which process is characterized in that a compound having the general formula H.sub.2 C.dbd.CR.sup.1 -COOR.sup.2, wherein R.sup.1 represents H or a linear alkyl group having from 1 to 3 carbon atoms and R.sup.2 represents a linear alkyl group having from 1 to 3 carbon atoms, is reacted in a solvent at a temperature of from -50.degree. C. to +50.degree. C. in the presence of nickel compounds as catalysts which contain complex anions X in a molar ratio Ni:X=1:1 and organic or hydride ligands and are modifed with phosphanes having the general formula PR.sub.2 R', wherein R and R' may be same or different and represent alkyl residues having from 1 to 8 carbon atoms or phenyl residues, as well as the use of the dimers formed in the dimerization.