AbstractBead macroporous terpolymers of methacrylaldehyde with methyl methacrylate and ethylene dimethacrylate were prepared. Preparation conditions (composition of the monomer mixture, of the inert components, concentration of the initiator and of the suspension stabilizer, and the rate of stirring) were discussed with respect to the properties of the terpolymers (porosity and concentration of reactive aldehyde groups).Concentration of reactive groups in the polymer was controlled by adding a third monomer. The analytically determined differences in the concentration of the reactive groups in the batch and in the polymer were interpreted (a) by the inaccessibility of the groups within the polymer bulk (for polymers with 30–50 mol‐% of crosslinking agent), (b) by the presence of tetrahydropyran rings or acetate bonds between the chains, and (c) by the formation of extractable soluble polymers or copolymers, or by not reacting of parts of the monomers (observed predominantly with low‐crosslinked polymers containing 10 mol‐% of crosslinking