Ethyl(hydroxyethyl)cellulose was functionalized with Brooker's merocyanine. The modified polymer was easily transformed in a film, which could be used as a highly selective chromogenic and fluorogenic chemosensor for the detection of cyanide in water, with detection limits of 1.9 × 10(-5) and 1.0 × 10(-7) mol L(-1). The film was successfully applied to the detection of cyanide in cassava (Manihot esculenta Crantz) roots, which are a well-known source of endogenous biological cyanide.