Abstract A simple flow-injection (FI) spectrofluorimetric method for the assay of total volatile aldehydes in auto exhaust gas and emission gas from thermal degradation was developed. Aldehydes, such as formaldehyde, acetaldehyde, propionaldehyde and n-butyraldehyde, reacted with cyclohexane-1,3-dione (CHD) to form more strongly fluorescent compounds. A two-channel flow system was assembled. Distilled water and 0.02% CHD were delivered at 0.75 ml min −1. The optimum conditions were pH 5 (2.2 M CH3COONH4-CH3COOH buffer solution), reaction temperature 70 °C, reaction coil length 0.5 mm i.d. × 7 m, cooling coil length 2 m, sample size 60 μ1, excitation and emission wavelengths, 376 nm and 452 nm. Aldehydes in sample gas (10 1) were collected by passing the gas at a flow rate of 0.5 1 min −1 through two impingers connected in series. 10 ml of methanol was used as an absorbent and diluted sample solution was injected into the carrier stream. The calibration graph was linear in the range 100–1000 ppb. The detection limit was 30 ppb and a sampling frequency of 30 h−1 was attained. Relative standard deviation for 10 standard formaldehyde solutions (500 ppb) was 1.5%. This rapid and simple FI method was applied to the determination of the total amount of aldehydes, calculated as formaldehyde, in auto exhaust gas and emission gas from the thermal degradation of polymers. The method is useful for monitoring aldehyde emissions and investigating the removal effect of aldehydes from various sources.