Abstract The isomeric cross-section ratio has been measured for Br 80 produced firstly by the Br 81 ( n, 2 n) Br 80 reaction with 14·7 MeV neutrons and secondly by the Br 79( n, γ) Br 80 reaction with 2·5 MeV neutrons. The accuracy of the measured results is enhanced by calculating the isomeric cross-section ratio entirely from the decay of the ground state of Br 80. The results are compared to the predictions of the statistical model of nuclear reactions and it is found that both reactions produce a consistent value for the spin cut-off parameter, which determines the spin dependance of nuclear level densities. The isomeric cross-section ratio is found to be 1·21 ± 0·09 for the Br 81 ( n, 2 n) Br 80 reaction and 0·49 ± 0·0.2 for the Br 79( n, γ) Br 80 reaction. The value produced for the spin cut-off parameter is 2·7 ± 0·1.