Polyelectrolyte complex membrane formation has been studied, online, using the synthetic boundary technique of an Optima XL-I anal. ultracentrifuge. This recent method, established by the first author, has now been evaluated concerning its sensitivity to monitor differences in the membrane formation if the membrane components and exptl. conditions are varied. For this purpose the membrane formation of sodium alginate with three polycations, poly(vinylamine)hydrochloride, poly(L-lysine)hydrochloride, and chitosan oligomers, was examd. under various exptl. conditions. In addn., two approaches have been developed and applied to evaluate the membrane formation process and the membrane quality on the basis of the exptl. received radial absorption scans. The first approach generalizes the membrane growth and its direction, resulting in a time-dependent description of the membrane geometry. The second one considers the membrane thickness as a function of time and might, therefore, serve to calc. the growth velocity or shrinking of the membrane, but not the directions of growth or shrinking. The investigations revealed a sufficient sensitivity of the method leading to the conclusion that the membranes formed in this study differ in thickness, symmetry, homogeneity, and the kinetics of formation. Consequently, the demonstrated sensitivity of this ultracentrifugation technique offers novel extended possibilities for basic studies of the polyelectrolyte complex membrane formation. Moreover, the goal-directed optimization of such membranes for practical applications can be supported. [on SciFinder (R)]