A photoactivated time-temperature indicator is based on a leuco base system. A thermally insensitive, white ("inactive") leuco base (or a mixture of such leuco bases) is mixed, preferably in a polymeric matrix, with a material that generates acid upon exposure to light. Photoexcitation, preferably by UV or near UV light, causes the formation of a thermally sensitive, color-forming ("active") product. 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 subambient temperature.