In this work, the influence of the different ionic strengths in the aqueous subphase on the monomerdimer equilibrium of a water-soluble tetracationic porphyrin, TMPyP, in a complex monolayer containing an insoluble lipid with negatively charged group, DMPA, has been investigated. LiCl, KClO4, and tetraethylammonium perchlorate, TEAP, have been used. Surface pressure-area, pi-A, and surface potential-area, Delta V-A, isotherms have been measured in the presence of salts in the subphase. As reference, the monolayer of DMPA in absence of TMPyP was studied by pi-A and Delta V-A isotherms on those subphases. Reflection spectroscopy, Delta R, was also used to infer the arrangement of the porphyrin in the cospread monolayer of TMPyP/DMPA, molar ratio 1:4, on different aqueous salt solutions. A decrease of Delta R with increasing salt concentration independent of the type of salt is obtained indicating a loss of porphyrin molecules to the subphase, which magnitude is dependent on the electrolyte : Further, a slight red shift of lambda(max) with respect to that at 420 nm obtained in absence of salt is observed. The analysis of reflection spectra at the air-water interface in the presence of Li+ and K+ ions in the subphase proposes a new semidimer phase for the porphyrin molecules in the mixed monolayer with a dimer fraction of alpha(D) approximate to 0.5. The model cannot be applied to the effect of TEAP molecules on the cospread monolayer due to the big size of TEA(+) ion.