A model for deposit formation in suspension firing of biomass has been developed. The model describes deposit buildup by diffusion and subsequent condensation of vapors, thermophoresis of aerosols, convective diffusion of small particles, impaction of large particles, and reaction. The model describes particle sticking or rebound by a combination of the description of (visco)elastic particles impacting a solid surface and particle capture by a viscous surface. The model is used to predict deposit formation rates measured during tests conducted with probes in full-scale suspension-fired biomass boilers. The rates predicted by the model were reasonably able to follow the rates observed in the tests, although with some variation, primarily as overestimations of the deposit formation rates. It is considered that the captive properties of the deposit surface are overestimated. Further examination of some physical parameters related to the description of surface capture are suggested. On the basis of these examinations of the model's ability to describe the observed deposit formation rates, the proposed model can be regarded as a promising tool for describing deposit formation in full-scale biomass suspension-fired boilers.