The pyrolysis of acetaldehyde has been investigated over a temperature range of 680-780℃ by a flow technique in the presence of excess toluene.Under our experimental condition,it has been shown that the pyrolysis of acetaldehyde is an unimolecular and first-order molecular decomposition reaotion,and its kinetic expression is k_ACH=1.22×10¹³exp(-68,000/RT)sec^-1(680～780℃) At the same time,we obtain the kinetic expression of the decomposition of toluene as k_ΦCH3=2×10¹³exp(- 77,000/RT)sec^-1(680～780℃)The results obtained lead to the conclusion that 11-12 vibrational degrees of freedom do not take part in the activation process,and the remaining 3-4 vibrational degrees of freedom do not take part in the activation process.