To precisely identify the effect of molecular weight of atactic poly(vinyl alcohol) (a-PVA) on the durability and polarizing efficiency (PE) of a-PVA/dye polarizing film, we prepared two (a-PVA)s with similar syndiotactic diad contents of 54.0%, degrees of saponification of 99.9%, and with different number-average degrees of polymerization [(P-n)s] of 1700 and 4000, respectively. Through a series of experiments, it was found that molecular weight of a-PVA had a significant influence on the durability of a-PVA/dye film in heat and humidity conditions (relative humidity of 80% and temperature of 90 degrees C). That is, both desorption of dye in a-PVA/dye film and transmittance of film decreased with increasing molecular weight of PVA. The change of PE (durability) of a-PVA/dye film in heat and humidity conditions was limited to about below 5%. The change of PE of PVA/dye film having P-n of 4000 especially was limited to 1%, whereas that of a-PVA/iodine film with P-n of 4000, was almost 80% under the same condition. Also, transmittance of the drawn a-PVA/dye film was far higher than that of the undrawn one. (c) 2005 Wiley Periodicals, Inc.