This study investigated the decomposition characteristics of acetaldehyde using an electron beam. The removal efficiency (RE) in air, O2, N2 and He atmospheres at 10 kGy were 88, 89, 94 and 35 %, respectively. By varying the initial concentration (C0), G-values at 240 ppm (C0) were maintained from 6.4 to 7.0 molecules/100 eV, while the G-values at 34 and 60 ppm (C0) decreased from 4.5 to 1.1 and from 6.6 to 2.0 molecules/100 eV when the absorbed dose increased from 2.5 to 10 kGy. The RE of acetaldehyde at 96 % relative humidity was approximately 10–15 % higher than that at dry air when the absorbed doses were 5–10 kGy. Increasing the water supply did not provide additional improvement of the RE at 2.5 kGy. CO, CO2, O3 and trace VOC compounds such as C2H4O2, C7H6O, C6H6, C7H8 and C8H10 were detected as by-products.