In this work, semi-interpenetrating polymer network (s-IPN) hydrogels of poly(vinyl alcohol) (PVA) with different contents of water-soluble sulfonated polyester (PES) were obtained by freezing and thawing cycles. The samples were characterized by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC). PALS was used to determine the average free volume radius through lifetime measures of the orthopositronium (o-Ps). Degree of crystallinity of the PVA/PES hydrogels was evaluated using the melting enthalpy ratios between the samples and the 100% crystalline PVA. The results show that an increase on the PES content leads to a decrease on the degree of crystallinity of the samples, reflecting an increase on the lifetimes (tau(3)). These structural changes could be interpreted as a result of different polymer-polymer interactions between PVA and PES in the hydrogels. (C) 2007 Wiley Periodicals, Inc.