A method for the assessment of the effect of excipients, pH and combinations thereof on the predicted absorption properties of low solubility compounds, the method comprising the step of assessing a change in a flux function for a combination of a low solubility compound and an excipient, with the flux function corresponding to particles or mass passing a barrier per area unit and time unit, wherein the barrier is selected from the group consisting of human tissues, animal tissues, plant tissues, cultured-cell models, and artificial membranes, and wherein assessing the change of the flux function comprises the steps of: (a) preparing a calibration donor solution at a predefined pH value comprising the compound; (b) preparing a donor solution at the predefined pH value comprising the compound and the excipient; (c) preparing a receiver solution at the predefined pH value free of the compound and the excipient; (d) providing the calibration donor solution in a first donor chamber being separated from a first receiver chamber by the barrier, providing the donor solution in a second donor chamber being separated from a second receiver chamber by the barrier, and providing the receiver solution in the first and the second receiver chambers; (e) incubating said solutions for a predefined period of time; and (f) measuring the response of the compound in the receiver solution of the first and the second receiver chambers, wherein measuring the response of the compound in the receiver solution of the first and the second receiver chambers comprises the measurement of spectroscopic properties of the receiver solution, repeatedly performing steps (a) to (f) at a plurality of predefined pH values, and determining a ratio between the respective measurement of the response of the compound in the receiver solution of the first receiver chamber and the corresponding measurement of the response of the compound in the receiver solution of the second receiver chamber. (a) preparing a calibration donor solution at a predefined pH value comprising the compound; (b) preparing a donor solution at the predefined pH value comprising the compound and the excipient; (c) preparing a receiver solution at the predefined pH value free of the compound and the excipient; (d) providing the calibration donor solution in a first donor chamber being separated from a first receiver chamber by the barrier, providing the donor solution in a second donor chamber being separated from a second receiver chamber by the barrier, and providing the receiver solution in the first and the second receiver chambers; (e) incubating said solutions for a predefined period of time; and (f) measuring the response of the compound in the receiver solution of the first and the second receiver chambers, wherein measuring the response of the compound in the receiver solution of the first and the second receiver chambers comprises the measurement of spectroscopic properties of the receiver solution, repeatedly performing steps (a) to (f) at a plurality of predefined pH values, and determining a ratio between the respective measurement of the response of the compound in the receiver solution of the first receiver chamber and the corresponding measurement of the response of the compound in the receiver solution of the second receiver chamber.