The well-known pH-indicators, phenolphthalein and bromothymol blue, exhibit an outstanding thermochromism, when they are embedded in aqueous polyvinyl alcohol/borax gel networks. The color of phenolphthalein in hydrogels changes gradually from colorless at 20degreesC to dark red at 70degreesC. The pH change in a hydrogel system was from 8.0 to 8.9 with increasing temperature. In the case of bromothymol blue, the color was optically clear green at room temperature and changed to clear blue with increasing temperature. (C) 2003 Wiley Periodicals, Inc.