Fluorescent polyanthracene prepared by a chemical oxidative route was shown to sense ferric ions (Fe³⁺) in aqueous acidic solutions. The fluorescence titration indicates that the intensity of fluorescence of the polyanthracene (PAT) was gradually quenched by the successive addition of an Fe³⁺ ion solution (‘Off-fluorescence’). The fluorescence quenching data fit well into the conventional Stern–Volmer relationship. A mechanism of Photoinduced Electron Transfer (PET) is proposed to explain the observed ‘Off fluorescence’ state of the PAT. Based on the strong affinity of Fe³⁺ to F⁻ ions, the mixture of polyanthracene–F⁻ ions exhibiting ‘On fluorescence’, could behave as a fluorescent ‘On type probe’. Thus a method using a single probe for the detection of both Fe³⁺ and F⁻ ions is developed using a fluorescence ‘Off–On’ approach, and is successfully applied for the determination of these ions in commercial samples.