A novel colorimetric chemosensor containing terpyridine was synthesized. It showed high selectivity and sensitivity for Fe(II) and Fe(III) ions in neutral aqueous solution in the presence of other metal ions such as Cd²⁺, Cr³⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Pb²⁺, Na⁺, Ca²⁺, Mg²⁺, K⁺ and Ag⁺. Upon the addition of Fe²⁺ or Fe³⁺, the sensor displayed a unique new peak around 567 nm in its absorption spectra, and the color of the solution changed from light yellow to light magenta. In particular, the respective amounts of Fe(II) and Fe(III) ions in the solution can be detected using the light-absorption ratio variation approach (LARVA).