Apparent diffusion coefficients, D-app,D-i, were determined in solutions with a polycationic additive - poly(diallyldimethylammonium) - acting as a pseudostationary phase for electrokinetic chromatography. They were determined for six small neutral analytes at five concentrations of the polymeric additive (between 0 and 4% w/w) by a stopped migration method. The apparent diffusion coefficients decrease with increasing polymer concentration only within 40% maximum, an effect that cannot be associated with the macroscopic viscosity of the polymer solution (which increases by a factor of 10). The change of the apparent diffusion coefficients is related to the interaction of the neutral analyte molecules with the polyelectrolyte chain. Applying the model of analyte partitioning between "free" solution and polymer, capacity factors and partition constants were derived from the slope of the 1/D-app,D-i vs. polymer concentration curves. Partition constants determined by this method (ranging between 40 and 170) agree with those obtained by electrokinetic chromatography.