A study on the combustion characteristics of Salix viminalis, a fast growing willow, was conducted at a 12 MW circulating fluidized bed boiler. The purpose of the study was to increase the understanding of the mineral matter behavior in the boiler and to foresee possible bed agglomeration or slagging and fouling problems that may occur during the combustion of this type of fuel. Special focus was given to the impact of ash chemistry on the slagging, fouling, and bed agglomeration. Samples from all ingoing (bed material, fuel) and outgoing solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg were collected and analyzed chemically. Selected bed samples and ash samples were also analyzed with a scanning electron microscope (SEM/EDAX). Deposit samples were collected at the cyclone inlet and from two different locations in the convective path using specially designed surface temperature-controlled deposit probes. All collected probe deposits were photographed and characterized visually. Selected samples from both windward (front) side and leeward (back) side of the sampling probes were analyzed chemically as well as with SEM/EDAX. In addition to these samples, the boiler operation was monitored carefully. This included collection of operational data (fuel feed, air distribution, and total air), collection and monitoring of pressure drops in the furnace, flue gas temperature profiles, and emissions. Multicomponent multiphase thermodynamic equilibrium calculations were then performed for predictions of the fly ash thermal characteristics, using the fly ash chemical composition as input data. The thermal characteristics, i.e., the melting behavior, were predicted for the different ash samples and compared with the results from the full scale fouling measurements. The paper discusses the impact of the ash chemistry on the bed agglomeration and fouling tendency found during the combustion tests and draws conclusions about their relevance to the operation of the boiler.