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This paper reports on the development of a new technique for inexpensive measurements of SO2 profiles using a modified dual-ozonesonde instrument payload. The presence of SO2 interferes with the standard electrochemical cell (ECC) ozonesonde measurement, resulting in –1 molecule of O3 reported for each molecule of SO2 present (provided [O3].[SO2]). In laboratory tests, an SO2 filter made with CrO3 placed on the inlet side of the sonde removes nearly 100% of the SO2 present for concentrations up to 60 ppbv and remained effective after exposure to 2.8 3 1016 molecules of SO2 [equivalent to a column ;150 DU (1 DU 5 2.6931020 moleculesm22)]. Flying two ECC instruments on the same payload with one filtered and the other unfiltered yields SO2 profiles, inferred by subtraction. Laboratory tests and field experience suggest an SO2 detection limit of ;3 pbb with profiles valid from the surface to the ozonopause [i.e., ;(8–10 km)]. Two example profiles demonstrate the success of this technique for both volcanic and industrial plumes.