Gelatin is well suited in the preparation of nanosize particles by precipitation of organic substances. Due to their amphiphilic structure, the gelatin molecules exhibit a pronounced surface activity. The surface activity was investigated by means of the Wilhelmy plate and the maximum bubble pressure technique within a time frame of more than five orders of magnitude. The influence of an emulsifier and the gelatin/emulsifier interaction are discussed. By means of laser Doppler electrophoresis, the electrophoretic mobility of A-type and B-type gelatin is measured. The molecular weight distribution of gelatin is measured by size exclusion chromatography. The size of dissolved and adsorbed gelatin molecules is determined by means of dynamic light scattering. It is shown that the surface activity of the gelatin/emulsifier complexes controls the size of the precipitated particles. Furthermore, we show that gelatin is adsorbed by hydrophobic interactions and provides excellent colloidal stability to nanosize particles.