The nuclear reactions 25Mg(α,n)28Si, 26Mg(α,n)29Si and 18O(α,n)21Ne are investigated in the astrophysically interesting energy region from Eα = 1000 keV to Eα = 2450 keV. The experiments were performed at the Nuclear Structure Laboratory of the University of Notre Dame (USA) with the Van-de-Graaff accelerator KN. Solid state targets with evaporated magnesium or anodized oxygen were bombarded with α-particles and the released neutrons detected. For the detection of the released neutrons, computational simulations were used to construct a neutron detector based on 3He counters. Because of the strong occurrence of background reactions, different methods of data analysis were employed. Finally, the impact of the reactions 25Mg(α,n)28Si, 26Mg(α,n)29Si and 18O(α,n)21Ne on stellar nucleosynthesis is investigated by means of network calculations.