A Taylor dispersion method was used to measure quaternary diffusion coefficients (D(11), D(22), D(33), D(12), D(13), D(21), D(23), D(31) and D(32)) of the four-component system beta-cyclodextrin KCl + caffeine + water at 298.15 K at carrier concentrations from (0.002 to 0.010) mol.dm(-3) for each solute with an inaccuracy of +/- (1 to 2) %. These diffusion coefficients were measured in order to obtain a better understanding of the structure of these systems, e.g., for reliable and safe delivery of precise dosages of medical drugs.