An improved time-temperature indicator (t-T indicator) is described which is useful for monitoring the shelf lives of various perishable articles. The device is constructed of a vapor-permeable barrier positioned between a vapor and a liquid indicator, both housed in a vapor-impermeable container. The device is activated just prior to the monitoring period by providing vapor to the container, as for example, by rupturing a solvent-filled frangible reservoir. The liquid indicator, containing a polydiacetylene, upon contact with the vapor or condensed vapor produces a visual color response, and the vapor must permeate through the vapor-permeable barrier before contacting the liquid indicator, thus creating a characteristic induction period before a color response occurs. Color response occurs substantially simultaneously over the entire observable surface of the liquid indicator upon contact of the liquid indicator with the vapor. The time required for the color response to occur in the device is a function of the temperature and time. The induction period can be varied by the thickness and type of material of construction used for the vapor-permeable barrier, the nature of the vapor and the nature of the liquid indicator employed.