Gaseous H 2 O 2 is sampled on a Ti(IV)-H 2 SO 4 coated filter, where titanium peroxy complex is formed. After sampling the filter is leached in a phosphate buffer (pH 7) and the peroxide concentration analysed spectrophotometrically at 505 nm after having added 4-(2-pyridylazo)resorcinol (PAR). The triple complex formed shows an extinction coefficient (3.5 10 4 1 mol - 1 cm - 1 ) about 2.5 times higher than that of the [FeNCS] 2 + complex, which is formed when the titanium complex is broken by addition of ferrous thiocyanate reagent. The detection limit of H 2 O 2 for 1-h sampling can be lowered to 0.03 ppb when this new reagent is used.