Polyelectrolyte multilayers are usually prepared from polydisperse polyelectrolytes. It is desirable to measure and control multilayer composition when it deviates from the deposition solution. With neutron reflectivity, multilayers prepared from PDADMA and binary mixtures of long deuterated PSSd(long) (80.8 kDa) and short protonated PSSd(short) (10.6 kDa) were investigated. A small amount of PSSd(long) in the deposition solution led to a disproportionate increase of PSSd(long) in the film, and completely suppressed the exponential growth regime. Adsorption kinetics were studied with in situ ellipsometry: (i) During adsorption of a PSS layer, the PSSlong fraction increased with adsorption time; PSS(short )could desorb, while PSSlong adsorbed irreversibly. (ii) During adsorption of a PDADMA layer, a fast thickness increase was followed by a slow thickness decrease. This was attributed to the formation of PDADMA/PSSshort complexes, which eventually desorbed. The desorbed thickness depended on the number of layers deposited. The mechanism of layer formation is attributed to the asymmetric growth of PDADMA/PSS multilayers, as supported by the adsorption kinetics of multilayers prepared from one kind of PSS.