A general method of preparation of thin-film sensors for O 2 , incorporating the dye ion-pair tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) ditetraphenylborate, in a variety of different thin film polymer/plasticizer matrices is described. The sensitivity of the sensor depends upon the nature of the polymer matrix and plasticizer. A detailed study of one of these systems utilising the polymer poly(methyl methacrylate), PMMA, is reported. The sensitivity of this O 2 sensor depends markedly upon the plasticizer concentration and is largely independent of temperature (24.5–52.5 °C) and age (up to 30 d). When exposed to an alternating atmosphere of O 2 and N 2 , a typical oxygen film sensor in PMMA exhibits a 0–90% response and recovery time of 0.4 and 4.5 s, respectively.