Abstract Chemical and isotopic analyses of aqueous species in co-produced waters from cyclic steam bitumen recovery wells at the Leming pilot, Cold Lake, Alberta, Canada, show the dominant control of co-produced water chemistry to be mixing of waters of natural origin with injected water and condensed steam. Co-produced waters from early cycles are characterized by highly transient compositions and temperature relative to later cycles which show more subdued trends with time. Production of injected water and condensed steam can be detected by freshening of co-produced water and interaction with a vapour phase can be detected by a decrease in δ 13C of dissolved carbonate with no concomitant decrease in the salinity of co-produced waters. Mixing diagrams with Cl as a conservative component have been constructed and are to show progressive mixing of relatively fresh injected water and condensed steam with more saline waters of natural origin. Mixing considerations imply the existence of at least two water of natural origin. One is produced typically in early cycles and is characterized by relatively high HCO 3 concentrations, molar Cl/HCO 3 ratios <∼5, and high δ 13C of dissolved carbonate species. The elevated δ 13C dissolved carbonate species and high HCO 3 concentrations of this bitumen-associated water indicate that its composition may have been modified by microbial degradation of original reservoired hydrocarbons and/or subsequent water-rock interactions of natural and artificial origin. A second water of probable natural origin is produced in later cycles. This water has intermediate TDS and molar Cl/HCO 3 ratios > 50 and probably originates in water-saturated sandstones which occur adjacent to the bitumen reservoirs.