Using neutron reflectometry we have resolved - to high resolution - the internal structure of self-assembled polyelectrolyte multilayer films and have developed a detailed molecular picture of such systems by analyzing the data with a composition-space refinement technique. We show that such surface films consist of stratified structures in which polyanions and polycations of individual layers interdigitate one another intimately. Nevertheless, the deposition technique leads to results that are predictable, if well-defined and constant environmental conditions are maintained during the preparation. For alternating layers of poly(styrenesulfonate) (PSS) and poly (allylamine hydrochloride) (PAH), adsorbed onto atomically flat surfaces, a roughening of successively deposited layers leads to a progressively larger number of adsorption sites for consecutive generations of adsorbed polymer, and thus to an increase in layer thicknesses with an increasing number of deposited layers. Because of the interpenetration of adjacent polyelectrolyte species, however, this increase settles quickly into an equilibrium thickness. In fully hydrated films (100% relative humidity), water occupies ≥40% of the volume within the films. About twice as much water (by volume) is associated with PSS as with PAH. Incorporated inorganic salt plays a minor role only, if any. The equilibrium thickness of the deposited layer structure may be fine-tuned via the ionic strength, I, of the solutions used for the preparation. We show that the dependence of the thickness d1p per layer pair on I is linear, with a sensitivity, Δd1p/ΔI = 16 Å × L/mol. Concurrently with the layer thickness the interface roughness σ between adjacent layers increases: σ ∼ 0.4 × d1p. In contrast to the ionic strength of the deposition solutions, the degree of polymerization of the polyanions used in the preparation plays a minor role only in determining the overall structure of the deposited films. The results reported here are quantitatively consistent with those of a recent study (Tarabia et al. J. Appl. Phys. 1998, 83, 725-732), if one assumes that the hydration of the polyelectrolyte molecules in the sample films investigated in the two studies is similar.@en