It has been found that ethylene-acrylic acid copolymers can be prepared by extruder cracking of ethylene-isopropyl acrylate copolymers. The cracking of the ester to the acid goes very smoothly to give 90-100% conversion; however, the melt index behavior of the product is quite complex. The melt index of the starting ester copolymer was 2.0, but the melt index of the thermally cracked product immediately after extrusion was only 0.1. On standing, the melt index of the product slowly increased to 30.0. This complex melt index behavior is related to anhydride formation and chain scission reaction in the extruder. The anhydride linkages give a pseudocross-linked structure and lead to an initial low melt index value. On standing, the anhydride groups are hydrolyzed with water from the atmosphere to give an uncross-linked acid copolymer. After the acid is formed, the main chain polymer scissions that have occurred during extrusion become evident and the high melt index value is observed.