This study examined the role of boric acid and the effect of heat treatment on PVA-iodine polarizing films prepared in the solution state before casting (IBC) of PVA/iodine/boric acid films. The films were prepared by casting aqueous solutions of 10 wt % poly(vinyl alcohol) (PVA) containing boric acid with 0, 0.1, 0.3, and 0.5 mol/l of I(2)/KI aqueous solution, and I(2)/KI(1 : 2) with 5 wt % of PVA. The effect of boric acid and heat treatment on the durability of the IBC PVA polarizing sheet films was investigated by UV-vis absorption spectroscopy. Boric acid was found to be essential for the complex formation in PVA/iodine solutions at relatively low I(2)/KI concentrations and high temperatures. The strength of the complex peak at similar to 600 nm in UV-vis absorption spectra increased with increasing boric acid concentration. With increasing heating temperature over 90 degrees C the intensity of the peak at 600 nm corresponding to the complex decreased due to the evaporation of I(2) decomposed from I(5)(-), but the peak at 355 nm corresponding to free I(2)center dot I(3)(-) was remained unchanged. From heat treatment at 150 degrees C, the intensity of the peak at 600 nm decreased but the intensity of the complex peak (600 nm) of the sample with 0.5 mol/l boric acid was unaffected. The transmittance and degree of polarization for the films increased and decreased with increasing heat treatment time under heat and a humid atmosphere, respectively. However, this tendency decreased with increasing boric acid concentration and heat treatment. (c) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1950-1956, 2011