To enhance durability of poly(vinyl alcohol) (PVA)/iodine polarizing film under humid and warm atmospheres and to identify the effects of syndiotacticity on the polarizing efficiency (PE) and durability of PVA/iodine complex film, we prepared three high molecular weight (PVA)s with similar number-average degree of polymerization (P) of 4000 and with different syndiotactic diad (s-diad) contents of 53, 56, and 59%, respectively. It was found that syndiotacticity of PVA had a significant influence on the durability of PVA/iodine complex film in warm and humidity conditions (relative humidity of 80% and temperature of 50 degrees C). That is, both desorption of iodine in PVA/iodine film and transmittance of film decreased with increasing syndiotacticity of PVA. In the case of PE, the values of over 99% were obtained at each optimum conditions. The change of PE (durability) of PVA/iodine complex films having P of 4000 and s-diad contents of 56 and 59%, respectively, in warm and humidity conditions was almost zero, whereas those of PVA/iodine film with s-diad content of 53% and with (P-n)s of 1700 and 4000 were about 60% and 50%, respectively, under same conditions. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 917-922, 2010