In this paper, we present a passive tag for the 1 determination of gaseous oxygen in intelligent packaging (IP). 2 The power supply for this tag is obtained from thermoelectric 3 energy harvesting taking advantage of the temperature difference 4 between a cooled package and the human body. For this purpose, 5 a compact Peltier module is attached to the surface of the pack-6 age. This device is able to generate 1.2 mW when a temperature 7 difference of 25°C is applied between its surfaces. A dc-to-dc 8 boost converter is included to generate an output voltage of 3.3 V 9 and an output current of 225 µA from the harvested energy 10 by the Peltier cell, which are used to supply the measurement 11 circuitry. A luminescent membrane sensitive to oxygen is used as 12 a gas detector in the package. The generated signal is compared to 13 a reference value to evaluate if the oxygen concentration inside 14 the package falls below or above a predetermined value. This 15 is shown by turning on a green or a red LED, respectively. 16 The proposed system presents a resolution of 0.02% of the 17 predicted oxygen concentration in the range of interest (0%-18 5%) and a limit of detection (LOD) of 0.007%, which makes the 19 instrument appropriate to be used in IP and active packaging 20 (AP) technology. 21 Index Terms-Intelligent packaging (IP), optical oxygen sen-22 sor, passive tag, thermal energy harvesting. 23 I. INTRODUCTION 24 O VER the last decades, the technology in food packaging 25 has been noticeably improved due to the growing interest 26 from the consumers in properties of the product such as 27 freshness, quality, and safety. Different approaches have been 28 followed in order to ensure the maintenance of the properties 29