The aim of this study was to achieve more understanding of the mass transfer characteristics of the filmadsorber haemoperfusion device. First, a structural model with mathematical description of the different diffusion steps was developed. Exact quantification appeared very difficult, resulting in insufficient fit of predicted and measured concentration curves. Moreover, the mathematics turned out simple, since the concentration courses could be described with one exponential power. Therefore, a formal model was developed, assuming linear isotherms and adsorption, proportional to the average concentration in the column. With this model predicted in-vitro inlet and outlet concentrations could be fitted to the measured data accurately. A relation between both models is given under the condition of high intraparticle mass transfer, which is allowed in case of powder adsorbents. It can be concluded that structural models do not yield predictive tools for optimization of device geometry. A formal model with two constants determining device performance enables device optimization with the help of some in-vitro experiments.