We describe an ozonosonde based on a homogeneous chemiluminescent reaction of olefins with ozone, allowing ozone measurements at all altitudes accessible to stratospheric balloons. The ozone–olefin reaction, which is bimolecular, gives a signal proportional to ozone mixing ratio. Typical response at 100 mb (∼16 km) with ethylene is 104counts s−1ppm−1. With 2‐methyl 2‐butene, one can extend the measurements to 1 mb. At 2 mb (43 km), we typically find 2×104counts s−1ppm−1. The time constant is 1 s, and the statistical noise of the reaction (∼1%) allows the use of the instrument to make accurate relative measurements for dynamical studies of the stratosphere. Two versions of the instrument have been flown successfully six times. We present a flight profile between 200 mb (∼12 km) and 12 mb (∼30 km), and also measurements at 2.5 mb (41 km) float level.