Adsorption of arginine-rich positively charged peptides onto neutral zwitterionic phosphocholine (PC) bilayers is a key step in the translocation of those potent cell- penetrating peptides into the cell interior. In the past, we have shown both theoreti- cally and experimentally that polyarginines adsorb to the neutral PC-supported lipid bilayers in contrast to polylysines. However, the comparison of our results with pre- vious studies showed that the results often do not match even at the qualitative level. The adsorption of arginine-rich peptides onto POPC may qualitatively depend on the actual experimental conditions where binding experiments have been performed. In this work, we systematically studied the adsorption of R9 and K9 peptides onto the POPC bilayer, aided by molecular dynamics (MD) simulations and fluorescence cross-correlation spectroscopy (FCCS) experiments. Using MD simulations, we tested a series of increasing peptide concentrations, in parallel with increasing Na+ and Ca2+ salt concentrations, showing that the apparent strength of adsorption of R9 decreases upon the increase of peptide and/or salt concentration in the system. The key result from the simulations is that the salt concentrations used experimentally can alter the picture of peptide adsorption qualitatively. Using FCCS experiments with fluorescently labeled R9 and K9, we first demonstrated that the binding of R9 to POPC is tighter by almost two orders of magnitude compared to that of K9. Finally, upon the addition of excess of either Na+ or Ca2+ ions with R9, the total fluorescence correlation signal is lost which implies the unbinding of R9 from the PC bilayer, in agreement with our predictions from MD simulations.