We performed surface hopping simulations of Al + H2O collisions by a direct semiempirical method, reproducing the conditions of previous beam-gas experiments.(1,2) We observed the formation of the HAlOH species, that dissociates to AlOH + H after a lifetime of about 0.6 ps. This species undergoes nonadiabatic transitions to its first excited state and is responsible for chemiluminescence in the visible range, while the Al-H2O complex emits in the infrared. The computed emission band in the visible is red-shifted with respect to the experimental one,(1,2) because of slight inaccuracies of the potential energy surfaces. However, collisions with more water molecules and exciplex formation with excited Al(S-2, P-4) atoms may also contribute to the short wavelength emission, as we show by accurate ab initio calculations.