This work was aimed at modeling of major gaseous emissions (NOx, SO3, SO2, CO2) from boiler units of a power plant firing (or co-firing) fuel oil and natural gas for variable operating conditions (load and load-related variables: excess air, flue gas recirculation, etc.). The emission rate of the pollutants for the co-firing was estimated for a particular boiler using these characteristics for the burning of each fuel in the boiler on its own and taking into account energy fractions (contributions) of fuel oil and natural gas to the boiler heat input. The gaseous emissions (in terms of emission concentrations, emission rates and specific emissions) from a 200-MW boiler unit firing low-S fuel oil and from a 310-MW boiler unit firing (or co-firing) medium-S fuel oil and natural gas were estimated and compared for 50-100% unit loads based on actual fuel properties and load-related operating variables of these units. Upper limit for the energy fraction of medium-S fuel oil was determined for the 310-MW boiler unit co-firing the two fuels with the aim to meet the national emission standard for SO2. (C) 2006 Elsevier B.V. All rights reserved.