An activatable time-temperature indicator consists of an oxygen-sensitive reaction product of a triarylmethane dye and a decolorant that is coated on a substrate and covered by an oxygen barrier. Optionally, an oxygen-permeable layer is between the coating and the oxygen barrier. The indicator is activated by removing the oxygen barrier. Following this activation step, a progressive color development occurs at a rate that increases with temperature. The indicator is useful for monitoring the freshness of perishable products, particularly those stored at sub-ambient temperature.