AbstractCellulose acetate carbamates (CACs) are the polymers which result when organic isocyanates are reacted with the free hydroxyl groups of cellulose acetate (CA). CACs are more hydrolytically stable and exhibit physical properties which are superior to those of their CA mixed ester analogs. Two synthetic approaches to CACs have been utilized in this study: (1) preformation, i.e., separate synthesis of such polymers prior to their inclusion in solutions for membrane casting; and (2)in situformation, i.e., the inclusion of blocked isocyanates in standard dry process casting solutions of CA followed by thermal activation of the resultant dry membranes leading to regeneration of free isocyanate and subsequent CAC formation. Preformed CAC polymers have been prepared utilizing phenyl‐, 3‐chloropropyl‐, 3‐bromopropyl‐, and 3‐bromopropyl‐(isothio)‐, isocyanates. Polymers containing omega‐halocarbamate moieties were quaternized with dimethylbenzylamine to produce ionogenic (QCAC) polymers containing quaternary ammonium groups. DRY‐RO membranes from the QCACs exhibit flux/rejection values varying between 6–8 gfd at 98% rejection and 20 gfd at 90% rejection (0.5% NaCl feed at 400 psi and 25°C).In situformation of CAC membranes has been effected with tolylene‐ and hexamethylene‐diisocyantes, with quaternized isocyanate monomers employed for the preformed CAC polymers, and with specially tailored diisocyanates containing ionogenic groups. Crosslinking rendered all of the membranes acetone insoluble. Inasmuch asin situformation substitutes the easy synthesis of blocked isocyanate monomers for the more difficult separate synthesis of preformed CAC polymers, it is anticipated that the form