Using the first-principle density functional calculations, the equilibrium geometries and electronic properties of anionic and neutral aluminum-sulfur AlnSm (2≤n+m≤6) clusters have been systematically investigated at B3PW91 level. The optimized results indicate that the lowest-energy structures of the anionic and neutral AlnSm clusters prefer the low spin multiplicities (singlet or doublet) except the Al2‾, Al2, S2, Al4 and Al2S4 clusters. A significant odd-even oscillation of the highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps for the AlnSm‾ clusters is observed. Electron detachment energies (both vertical and adiabatic) are discussed and compared with the photoelectron spectra observations. Furthermore, a good agreement between experimental and theoretical results gives confidence in the most stable clusters considered in the present study and validates the chosen computational method. In addition, the variation trend of chemical hardness is in keeping with that of HOMO-LUMO energy gaps for the AlnSm clusters. Upon the interaction of oxygen with the stable AlSm‾ clusters, the dissociative chemisorptions are favorable in energy. The binding energy and Gibbs free energy change show completely opposite oscillating behaviors as the cluster size increases.