The table presents computed values of effective threshold energies, Eeff, for (n, 2n) reactions induced in a fission neutron spectrum. Eeff-values are given for the (n, 2n) reactions for which the charge of the target nucleus is greater than 11 and for which the reaction product cannot be formed by (n, γ) reaction on another stable isotope. A closed expression is given for the integral of a fission neutron distribution of the Cranberg type, N(E), and for the mean energy of the fission neutrons with spectral distribution N(E).