Abstract: A process for the preparation of semipermeable membranes with improved acid and base stability including: (a) coating a base and acid stable ultrafiltration (UF) membrane support polymer, with a high molecular weight amphoteric polyelectrolyte coating polymer optionally in solution, the amphoteric polyelectrolyte containing both cationic and anionic groups and in addition primary and secondary amine groups, the solution optionally containing a latent internal crosslinker compound that is a polyfunctional compound that crosslinks the coating polymer only during a curing step at elevated temperatures and basic pH; (b) draining or washing the coated support; and (c) reacting the coating with a solution of an external crosslinking compound that is a polyfuntional compound, not overly reactive so as not to be hydrolyzed prematurely in solution, the time of exposure to the external crosslinking compound being sufficient to ensure its diffusion into the bulk of the coating, so that the crosslinking is effected not o