Extraction of petroleum from the oil sands creates lar ge quantities of oil sands process- affected water (OSPW) and tailings sand. The tailings sand has a low fertility, a low organic matter content, it is highly alkaline, compactable, and contains residual hydrocarbons, making it a very inhospitable growth environment. As the exploitation of this resource intensifies, increasing quantities of OSPW and tailings sand are being produced. The petroleum industry is currently involved in efforts to revegetate and remediate the tailings sand, and one approach is to revegetate the tailings sand with Frankia-inoculated alders. Alders are primary successor trees that have the ability to grow in nutrient poor and waterlogged environments, in part because they form a symbiotic relationship with the nitrogen-fixing actinomycete, Frankia. The effect of Frankia-inoculated alders on soil quality was evaluated by monitoring the chemical, physical and microbiological characteristics of soil (organic matter, pH, bulk density, salts and nutrients concentrations, microbial population density, activity and diversity). The impact on the indigenous microbial community was also studied using hydrocarbon mineralization assays, molecular biology tools such as denaturing gradient gel electrophoresis (DGGE), and catabolic gene probing, using genes from bacterial pathways for the degradation of various hydrocarbons. Plant parameters were measured to evaluate the impact of Frankia on alder health and growth (plant biomass, nitrogen content, plant height). Preliminary greenhouse trials demonstrated the immense potential of this approach. In 2005, field trials were established at Syncrude Canada Ltd. After two growth seasons, samples of the bulk soil showed an increase in hydrocarbon (hexadecane, naphthalene and phenanthrene) mineralization where the tailings sand had been planted with Frankia-inoculated alders compared to both unplanted and planted with non- inoculated alders. The rhizosphere samples all had comparable hydrocarbon mineralization rates. Soil tests showed that alders inoculated with Frankia decreased the soil pH, increased buffering capacity and decreased the percent saturation in three main soil cations (Ca, K, and Mg). The field results have confirmed that the alder-Frankia combination results in improved remediation capabilities and improves soil quality. These improvements in the quality of the tailings sand will allow the subsequent establishment of more sensitive species, leading ultimately to the reforestation of the site.