We report on ellipsometric studies of the nonlinear 600 growth regimes of electrostatically assembled polyelectrolyte inultilayer films under in situ conditions. Poly(styrenesulfonate) (PSS) and poly(aiallylinethylarnmonium) (PDADMA) with different molecular weights in 0.1 mol/L NaCl are :used. Always, the linear, growth- is preceded by a parabolic growth regime. For films made from binary PDADMA mixtures, the compositions in the film and the adsorption solution are nearly identical. To explain these findings, a molecular model that describes the parabolic growth quantitatively is introduced. Neutron reflectivity of films prepared from PSS nioleCular weight above a threshold, >25 kDa, shows that its center of mass is vertically immobile, in-both the parabolic and linear growth regimes. Below 25 kDa, the multilayer buildup starts with an exponential growth regime, and pronounced PSS interdiffusion is found with neutron reflectivity. The exponential buildup regime is extended on decrease of PSS molecular weight. For films made from binary PSS mixthres, addition of less than 1 mol % PSS with molecular weight above the threshold, 25 kDa, prevents the: formation of an exponential growth regime and addition of 10 mol % suppresses the influence of low molecular weight PSS on film buildup.