Optically indicating surface de-icing fluids

An optically indicating de-icing solution for surfaces comprising a freezing point depressant liquid and a compound which exhibits a visually observable change as solid phase domains become present in the solution is disclosed. When applied to a surface, particularly the surface(s) of aircraft, the formation of solid phase ice domains in the liquid provides a distinct and visible change in the appearance of the solution. This allows a determination of ice formation upon the aircraft as well as a determination of the effectiveness of the de-icing solution.