A new and simple chemical method for deposition of thin blue films composed of Prussian Blue and N-substituted polypyrroles on nonconductive supports is presented. It is found that only pyrroles which are difficult to polymerize can be used for the preparation of such films. The resulting composite films were examined by SEM-EDAX, vis−NIR, and IR spectroscopy. The films are stable, thin, homogeneous, and optically transparent. The absorption maxima are at 720 nm, and spectral changes can be monitored using semiconductor light sources and detectors. The composite films are shown to be suitable for optical determination of pH over the pH 5−9 range because their absorbance strongly depends on pH in the physiological pH range. However, they undergo irreversible spectral changes if exposed to pH's >9. The films represent an alternative to indicator-based pH sensor materials because they do not require a dye to be immobilized. The pH measurements are highly reproducible, reversible in the physiological range, and not affected by ionic strength, alkaline cations, and typical oxidants and reductants.