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Physicochemical characteristics of the soot deposits in a fouled EGR cooler are studied in this paper. It is found that a three-layer model for the soot deposited in the EGR cooler may well describe the behavior of the depositing process: a dense base layer with micro pores (≺ 5 nm), a randomly packed intermediate layer with meso pores (5-50 nm) and a loose surface layer with macro pores (≻ 50 nm). The surface layer is thick and highly porous, formed by mechanical interlocking of the agglomerated primary soot particles or soot clusters. The soot particles in the surface layer may be removed by a high shear EGR flow. Condensates in the deposit, especially water, can have a significant influence on the structure of the deposit. Capillary forces on the wetted soot particles could be comparable to the contact forces holding the particles together. It is found that the hydroscopicity of the soot particles vary with their content of soluble organic fraction (SOF).