Surface‐attached, photo‐crosslinked gel films of a N‐isopropylacrylamide copolymer were investigated in ethanol/water mixtures using a combination of surface plasmon resonance/optical waveguide spectroscopy with reversed WKB analysis. The solvent quality of the pure good solvents drops in their mixture and this co‐nonsolvency effect shifts the transition temperature (Tc) in the µm‐thin gel films from 32.8 °C in pure water to 29.7 °C with only 0.25 vol.‐% ethanol. Between 20 and 70% ethanol and >10 °C (the practical temperature limit) the layers existed only in the collapsed state. A reentrant Tc of 40.2 °C was found at 70% ethanol while at higher ethanol volume fraction no Tc could be recorded.