Polyelectrolyte multilayers (PEMs) from poly(allylamine hydorchloride) (PAH) and sodium poly(styrenesulfonate) (PSS) are prepared at different conditions and investigated in air with X-ray reflectivity. Beyond a critical temperature the thickness per deposited polycation/polyanion bilayer and the surface roughness increase on heating the preparation solution. With increasing NaCl concentration that critical temperature decreases (40 °C at 1 M down to 15 °C at 3 M). Below the critical temperature the film thickness shows no dependence on the deposition temperature. Also, for ion concentrations up to 3 M, the surface roughness is constant (1−1.5 nm). However, for films prepared at 4 M the roughness increases on cooling, which is attributed to approaching the UCST at very high salt concentrations. AFM measurements in air show that the increased surface roughness is due to a random pattern of holes, whose separation is characterized by a single length scale, a behavior typical for spinodal decomposition. The length scale exceeds 100 nm and depends on the preparation conditions. Grains consisting of polyelectrolyte complexes with a size below 30 nm do not influence the surface roughness. It is suggested that the decomposition occurs during film drying since the film morphology in water shows no decomposition pattern, even when the film is cooled.