Permeable bioreactors have gained both research and management attention as viable methods for treating mine runoff waters. We examined the operation of a field-scale bioreactor (containing mixed compost, straw and gravel) for treatment of runoff from the Mother Load (ML) mine in northern Idaho, U.S. and compared it to an experimental laboratory-scale reactor, containing a similar matrix and treating similar mine runoff water. In general both reactors were efficient in removing most of the metals assayed, Al, As, Cd, Fe, Ni, Pb and Zn, with the exception of Mn. Both systems showed evidence of bacterial-mediated sulphate reduction and concomitant metal sulphide complexes. However, the experimental laboratory bioreactor showed greater proportions of immobile metals reductions than did the ML bioreactor, presumably due to the greater action of sulphate-reducing bacteria. The major metal removal mechanism in the ML bioreactor was surmised to be adsorption. Differences in metal removal mechanisms between the reactors were hypothesized to be due to fluctuating hydraulic residence times at the ML site, in turn, due to unregulated runoff flow.