The elusive protonated ozone ion (O3H+) has been long postulated as a reactive intermediate but never experimentally observed. This ion has been detected here in mass spectrometric experiments with the use of Fourier transform ion cyclotron resonance. In these experiments, ozone (O-3) was protonated by strong acids-for example, H-3(+), KrH1+ XeH+, and CH5+. The hitherto experimentally unknown proton affinity of O-3 was evaluated by a "bracketing" technique and determined to be 148 +/- 3 kilocalories mole(-1) at 298 kelvin, in excellent agreement with a value determined in a recent theoretical study of the O-3/O3H+ system, which was 148 kilocalories mole(-1) at zero temperature (similar to 149.5 kilocalories mole(-1) at 298 kelvin).