There is disclosed an indicator composition, the application thereof to substrates, and related products. The indicator composition comprises an organic solvent soluble polymer and a redox sensitive material which displays different visible properties in the oxidized and reduced forms. The organic solvent soluble polymer can be at least partially sulfonated polystyrene. The indicator composition can be dissolved in organic solvents such as acetone, ethanol and ethyl acetate to form inks which can be used in a variety of printing processes. The indicator composition can be used to detect oxidizing agents, oxygen, water, reducing agents, UV light, temperature and the passage of time.