Exciting light and neutron scattering as well as microscopic experiments have been performed recently with colloidal systems.1-6 Depending on the nature of the colloid particles and the experimental conditions, two- or three-dimensional supermolecular fluid-, glass-, and crystal-like ordered states have been obtained. Apart from fundamental insight into the structure and dynamics of colloidal systems, such investigations are often suitable to relate theory more directly to experiment than possible for atomic or molecular systems. In spite of the invaluable information obtainable from the above-mentioned experiments, significant lack in understanding the supermolecular structures remains as long as they cannot be related to the interaction between the constituent particles. We propose here to use the particle positions generated by Brownian motion for the determination of effective pair potentials U(r) by microscopy.