Poly(vinyl alcohol), PVA, and poly(acrylic acid), PAA, blends were prepared by solution casting. These polymers were found to be miscible in the whole composition range as determined by DSC even though some crystallinity remains in blends with PVA concentrations above 50wt%. Dynamic mechanical measurements of these blends and PAA as a function of temperature show an increase in storage modulus, E＇: when they reach a temperature of 140 degrees C that is well beyond their softening point. The E increase in PAA beyond 140 degrees C is attributed to an intramolecular reaction of cyclic anhydride formation that stiffens the chain. Isothermal storage modulus test as a function of time and FTIR measurements at 160 degrees C of PAA and three blends show that this increase in E＇ is due to cyclic anhydride formation.