Polycaprolactone (PCL) and Nafion blends were prepared in the presence of 3-aminopropyltriethoxysilane (3-APTEOS) by a sol-gel reaction. The presence of 3-APTEOS allowed the preparation of macroscopically homogeneous self-supported films of PCL/Nafion blends in a wider composition range. The thermal properties of the blends were evaluated by thermogravimetric analysis and differential scanning calorimetry. The thermal stability of the blends decreased as the Nafion content increased. The reduction of the thermal stability of PCL in the presence of Nafion was confirmed with Fourier transform infrared/photoacoustic spectroscopy, and it was attributed to the possible hydrolysis of PCL caused by the reaction with the SO3H acid groups of Nafion. An opposite effect on the thermal stability of the blends was caused by the addition of 3-APTEOS. The increase in the thermal stability of the blends with the increase in the 3-APTEOS content was assigned to the neutralization of the acid groups of Nafion by the formation of an ionic complex. The differential scanning calorimetry results showed that PCL and Nafion were immiscible. The degree of crystallinity of PCL slightly increased with an increase in the Nafion content, whereas the melting temperature remained basically unchanged. (C) 2004 Wiley Periodicals, Inc.