The study of reaction kinetics and process modeling plays a vital role in the development and optimization of industrial processes. In this study, a physicochemical model is presented for pressurized hot water extraction of spruce wood meal to facilitate process development and optimization. The model takes into account several relevant phenomena during the extraction process, including acidbase reaction equilibrium, irreversible reaction kinetics, ion exchange, and mass transfer. The system is modeled by assuming two liquid phases, i.e., liquid external to wood fibers and fiber-bound liquid to enable inclusion of ion exchange. The nonideal behavior of the system (due to presence of ions) is considered by utilizing activities instead of concentrations. The scission of hemicellulose polymers into oligomers and monomers is modeled by a population balance approach of discretized categories. Model parameters are optimized by fitting the model output to experimental data reported in the literature. The presented model aims to be the most comprehensive description of the phenomena taking place during the hot water extraction of softwood to date. Such physicochemical models are able to provide better understanding for parts of the process, which cannot be followed directly due to the tediousness of analytical methods. It also reveals gaps in the present knowledge and provides a step toward further development.