We present on the fly surface hopping simulations of the dynamics of photoexcited acetone in the n → π* band, taking into account both the spin–orbit and the dynamic couplings and allowing for the C–C bond dissociation. The S₀, S₁, T₁ and T₂ states were considered and the propagation time was 50 ps. According to the simulation results, after excitation to S₁ both Internal Conversion (IC) to S₀ and InterSystem Crossing (ISC) to T₁ or T₂ take place at comparable rates; T₂ plays an important role and the simultaneous treatment of the spin–orbit and dynamic couplings is shown to be mandatory to describe the photodynamics. We propose a mechanism that explains the observed fast and slow decay rates of the S₁ state of acetone.