We report salt-induced phase separation of polyelectrolyte complexes in solution correlates with deposition of polymer multilayers at a surface. Multilayer growth was followed in situ using FTIR-ATR (Fourier transform infrared spectroscopy by attenuated total reflection). The polymer system was quaternized poly(4-vinylpyridine) (QPVP) and poly(methacrylic acid) (PMAA), with various ratios of PMAA to QPVP chain lengths. In solution, phase diagrams were produced for QPVP/PMAA mixtures containing excess of PMAA or QPVP, in both cases showing salt-induced phase separation. These phase diagrams controlled deposition of PMAA or QPVP chains at the surface and explained a bell-shaped dependence of the multilayer thickness on the salt concentration. The order of chain removal (occurring at the step of QPVP or PMAA deposition) could also be predicted from the phase state of QPVP/PMAA mixtures in solution. Finally, we show that, by using short equilibration times with solution, robust multilayers can be produced at surfaces even in conditions when multilayer deposition is aggravated by desorption of polymer chains.