A new method for determining the osmotic pressure of electrostatically stabilized colloidal particles from frontal ultrafiltration experiments has been developed. The method is based on a reverse calculation utilizing a previously developed analysis of frontal ultrafiltration (W. R. Bowen and F. Jenner, Chem. Eng. Sci. 50, 1707, 1995; W. R. Bowen and P. M. Williams, Biotechnol. Bioeng. 50, 125, 1996). The method has the following advantages over conventional membrane osmometer measurements: (i) only very simple apparatus is required, (ii) results are obtained quickly, (iii) only small sample quantities are required, and (iv) only dilute initial samples are needed, as the ultrafiltration process creates a concentrated solution at the membrane, making sample preparation a simple task. A comparison of the osmotic pressure determined by this new method with previously measured osmotic pressure data shows excellent agreement. The approach demonstrates how analysis of process data may provide quantitative information on the interactions of concentrated colloidal systems.