The neutron-deficient isotopes257,258105 were produced in the reaction27Al+236u in 6n and 5n evaporation channels, respectively. The evaporation residues emerging from the target were separated in-flight from the projectiles and from products of different nuclear reactions by the electrostatic separator VASSILISSA . The isotopes were then implanted into position-sensitive silicon detectors and identified using the α-α-correlation method. The measured production cross-section is σ(5n)=(0.45±0.20)nb at E P =154 MeV and σ(6n)=(0.075±0.055) nb at E P =163 MeV. These cross-sections are compared with data measured for the same isotopes in the more symmetrical reaction50Ti+209Bi.