For the first time, the capacitive characteristics of a Pt-Ba0.25Sr0.75TiO3-Pt heterojunction structure sensitive to hydrogen peroxide vapor have been investigated in the temperature range of t = 36...42 °C and H2O2 concentrations of 0 % (distilled water), 0,5 %, 1 %, 1,5 %, and 3 %. It was found that the capacitance of the structure decreases with the increasing concentration of H2O2 in H2O2 + H2O and increases with the increase in temperature of vapor. A new sensing mechanism, which is mainly attributed to the decrease of dielectric permittivity of paraelectric thin film, is proposed for the explanation and revelation of experimentally observed dependences of capacitance. It is assumed that after chemisorption and heterogeneous catalytic decomposition of H2O2 vapor, high reactive oxygen species (ROS) are formed on the surface, which oxidize the paraelectric thin film, leading to the generation of new non-compensated oxygen vacancies. Due to the non-linear dependence of paraelectric dielectric permittivity on electric field, these charged oxygen vacancies, in turn, polarize the crystal locally, leading to a decrease in the permittivity of the paraelectric film and hence leading to a decrease in capacitance, which is observed during the experiments.