Polystyrene/maleic anhydride (PSMA) was synthesized to reach a viscosity-average molecular weight of 700 kDa and fabricated into ultrafine fibrous membranes consisting of fibers with an average diameter of 300 nm. These ultrafine PSMA fibers were rendered insoluble in organic solvents by reactions with hydrazine and ethylenediamine (ED). The highly efficient incorporation of diamines into the fibrous membranes was easily achieved by brief immersions in either dilute (0.5 wt %) hydrazine for 1 min or ED ether solution for 2 min. Heating at 150 degrees C for 5 min produced crosslinked PSMA with very little or no solubility in acetone with the retention of the fibrous membrane structure. The ED-crosslinked membranes were particularly stable to both bases and acids as well as hydrophilic solvents, had a 46 degrees water contact angle, and absorbed 22 times the amount of water as the as-spun fibrous membrane. This post-fiber-formation crosslinking approach was robust, highly efficient, and fast and required very little crosslinking reagent. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 723-730, 2010