Various types of ozone detectors are currently in use, each with different advantages and compromises in response time, portability, sensitivity, accuracy, need for repeated calibration, and expense. We describe here a new dual‐beam UV‐absorption instrument for balloon‐borne measurements of atmospheric ozone. It has two identical absorption chambers, each alternating between reference mode (ozone free) and sample mode by means of a four‐port valve and ozone scrubber. The ratio of the absorption signals, along with the known lengths and ozone absorption cross section, yield the ozone concentration. The dual‐beam feature cancels the effects of lamp intensity fluctuations, while the mode alternation compensates for mechanical changes and also provides continuous measurements. The absorption measurement requires no calibration and, hence, is independent of gas flow rate. The response time is 1 s and, for this measurement duration, the minimum ozone concentration detectable by this instrument (one standard deviation) is 1.5×1010 molecules/cm3 (0.6 ppbv at STP). The overall uncertainty of a 1‐s measurement at the ozone maximum (22 km) is 3.6%, where 2% of this is the accuracy of the ozone cross section. The size and weight are suitable for launch by small balloons, but the cost of the instrument precludes its use as a disposable unit.