In this paper, we investigated excitation functions of (n, 2n) reactions on the target nuclei in the mass region (144 <= A <= 160) for some samarium, europium and gadolinium isotopes. Studies were performed for incident neutron energies between 1 MeV and 20 MeV. We used the ALICE/ASH and TALYS 1.4 codes to investigate the pre-equilibrium effects and the ALICE/ASH were used for the equilibrium effects. The effects of collision probabilities on pre-equilibrium excitation functions were also investigated.