This work describes the detection of sulphur dioxide with a piezoelectric crystal, at a resonance frequency of 9 MHz, covered with an active coating containing trioctylmethylammonium dichromate which irreversibly bonds sulphur dioxide in an oxidation-reduction reaction. The decrease in the vibration frequency of the crystal is directly proportional to the mass of sulphur dioxide bonded. The detector works on the basis of integration of the decrease in the frequency over a given time interval (at least 10 min), resulting in a high measuring sensitivity. The calibration curve is linear for sulphur dioxide concentrations from a few to 500 microg/m(3). Partial exhaustion of the capacity of the active coating, occurring after prolonged use, appears as nonlinearity of the detector concentration response. This nonlinearity can be compensated by a simple mathematical correction that permits the lifetime of the active coating to be lengthened by several orders of magnitude.