The carbon dioxide concentration patterns in a large, high-rise, office building in Ottawa were examined experimentally using an automated data acquisition system. Daily C02 concentration profiles throughout the building and air change rates, using SF6 as a tracer gas, were measured at minimum outdoor air supply rates during much of a heating season. Of particular interest was how well mixed the indoor air was and how well the C02 concentrations that were measured in the central ventilation system's return air plenum represented the average C02 concentration behaviour in the building as a whole. C02 concentration profiles were also measured on individual floor spaces in the building to determine the range of variability in the concentration behaviour. The measurement results are presented and discussed in the context of demand-controlled ventilation.