Between 700 and 1100°C the electrical conductivity of undoped SrTiO3 changes from p-type to n-type as P(O2) decreases from 1 to 10-15 bar. For bulk materials the response time tR90 of the oxygen sensor upon changes of the oxygen partial pressure in the surrounding atmosphere decreases quadratically with the thickness of the sensitive material. Thick samples show a diffusion-controlled dependence of their response time. Thin films of about 1 μm exhibit response times in the range of a few milliseconds at temperatures of 1000°C. Comparing the activation energy with values given in the literature for bulk materials shows that for great changes of the oxygen partial pressure the response time of the sputtered films is controlled by the diffusion of the oxygen vacancies in the sensitive material. The lower activation energies for oxygen diffusion in and out of the sensitive material at above 800°C indicate an effect where the response time of the oxygen sensor is dominated by the reaction of the oxygen molecules on the surface of the sensitive material.