Low frequency internal friction measurements, over the temperature range from 100° K to 290° K, have been carried out on neutron-irradiated niobium. Two relaxation peaks, or groups of peaks, have been observed, one at about 110° K, and the other at about 210° K. These peaks have, respectively, been identified as α and β peaks observed in cold-worked bcc metals. The height of the α peak decreases and that of the β peak increases with increasing dose of irradiation within the order of magnitude of 10 18 nvt. On isochronal annealing (2 h pulses) the heights of the peaks have been found to increase over the temperature range from 65° C to 200° C, the increase in the height of the β peak being much more pronounced. This temperature range fits into the so-called Stage III annealing in bcc transition metals. On annealing at higher temperatures the heights of the peaks decrease to negligible values at about 380° C. The relaxation mechanisms causing the peaks have been discussed in terms of a dislocation (Bordoni type) process, a dislocation-point defect (Hasiguti type) process and point defect complexes. The general conclusion is that the α peak is caused by the processes involving dislocations and the β peaks are caused by processes involving point defects created by irradiation and interstitial impurities.