A mechanism of particles and condensable vapour deposition enabling simulation of the deposits composition and size has been developed. The results of deposit growth on heat transfer tubes during nine months of boiler firing by subbituminous coal containing 13.9 wt% of mineral matter are presented. It has been demonstrated that a 4 in deep front of most intensive deposition on the superheater tubes moves with time down the tubes bundle. The deposits grow quickly during the first three months of boiler operation, and then remain almost stable, reaching after nine months of operation a thickness of about 30 mm. (C) 2004 Elsevier Ltd. All rights reserved.