The aim of the study described in this article is to determine the effectiveness of fuel oil additives used in heavy duty conditions. The research has been conducted during real operating conditions of a steam boiler, belonging to a 320 MW power plant, in order to reveal fuel additive effectiveness in controlling flue gas emissions, plant availability, maintenance costs and operating efficiency. Portable instruments have been used to measure emission of pollutants during the testing period simultaneously with the installed monitoring equipment for continuous measurements. Adequate methods have been chosen and used to determine the corrosion rate, deposit grow-up, SO2 to SO3 conversion and the steam boiler efficiency. The results are presented graphically to compare effects of tested additives during four phases of examination in relation to the type of additive, steam boiler load and excess air for combustion. (C) 2012 Elsevier B.V. All rights reserved.