An experiment was initiated in 1997 in northeast Alberta at the Syncrude Canada Ltd. Mildred Lake site to field test an innovative technique for reclamation of oil sand mine tailings. This technique was used to create an aggregated soil material from oil sand tailings. A plant community was successfully established on the soil material created by this technique. However, whether the site would be capable of supporting a self-sustainable ecosystem for the long-term remained a challenging issue. We evaluated the capability of these aggregated oil sand tailings by using biological indicators of the abundance and diversity of soil microbial biomass. Soil respiration rates and soil microbial biomass measurements were used to assess the abundance and activities of soil microbial communities. The ability of soil microbial biomass to utilize a diverse range of carbon substrates was used to assess the diversity of soil microbial communities. Soil biological activity increased with increasing growth of plant biomass and over time. Increasing the amount of peat moss or muskeg incorporated into the soil during reclamation resulted in higher organic carbon and nitrogen content and caused an increase in abundance and diversity of soil microbial biomass. These results indicate that measurements of soil respiration and substrate utilization by soil microbial communities may be used as biological indicators for assessing the capability of reclaimed soils.