We have studied in situ the adsorption of a soluble anionic dye polymer onto spread monolayers by means of reflection spectroscopy. We found that the adsorption is governed by the charge neutrality; the ratio of the adsorbed polymer unit and the cationic monolayer molecule is nearly 1:1 for the surface density in the range of 50-77 angstrom2/monolayer molecule. On the other hand, no adsorption occurs onto an anionic monolayer. We have also investigated the adsorption kinetics of the polymer. The rate of adsorption can be reproduced very well with a simple diffusion simulation.