The invention relates to a process for immobilizing acidic or electrophilic compounds in a solution. The process is characterized in that it consists in bringing a crosslinked polyimine, which is insoluble in the medium and in which the imino groups are --NH-groups, into contact with said compounds and in then separating, by filtration, the modified polyimine obtained. The crosslinked polyimine comprises linear segments composed of --HN--R.sup.1--NH--R.sup.2--Ar--R.sup.3-units in which R.sup.1, R.sup.2 and R.sup.3 are identical or different alkylene or alkenylene groups and Ar represents an aromatic group. The process is of use for fixing undesirable compounds or for purifying acidic compounds.