The results of solids accumulation and deposition in the front and rear zone of some selected steel samples located in a 400 MW coal and biomass cofired CFB boiler are presented in this paper. The analysis was particularly focused on the determination of chemical composition of the deposits in some chosen sample areas (sample surface and cross-section). The investigation results indicated that the deposits on the front sections of the samples were rich in calcium and sulfur compounds, while increasing mass fractions of alkali, alumina and silica compounds were determined on sample rear zones. The morphology and topography of the deposits were investigated and the solids accumulated on the front section (i.e. directly exposed to the flue gas flow) revealed an oriented structure, while the deposits in the rear consisted mainly of the finest ash fractions and formed a smooth and homogeneous surface.