The imbibition process into the pore structure of highly porous nanoparticle layers is investigated. The layers are fabricated with the Flame-Spray-Pyrolysis process and consist of aggregated nanoparticles. Measurements of the pore structure reveal that these layers have a very high porosity and a broad pore size distribution. Imbibition experiments show a deviation of the imbibition process from the classical capillary rising theory (Bell, Cameron, Lucas and Washburn). Therefore, a new capillary rising model is developed that accounts for the complex pore structure within layers of aggregated particles through the implementation of the fractal dimension of tortuosity. Comparison of the new model and the performed imbibition experiments enable the determination of the equivalent pore diameter for the imbibition process. The resulting pore diameter agrees well with the mean pore diameter of the nitrogen physisorption measurements. Hence, the new model enables the prediction of the imbibition process into highly porous layers of aggregated particles.