The thermodynamics of ion paring between electrochemically reduced [(C6H13S)(8)PC](2)Lu or [(C12H25S)(4)PC](2)Lu and a cationic tetra-n-octylphosphonium bromide (4C(8)P(+)Br(-)) matrix that was deposited in a gel-like cast film on a basal plane pyrolytic graphite electrode was investigated in an aqueous KCl solution that varied in concentration from 0.005 to 1.0 M. The purpose of this study was to elucidate ion-pairing effects between reduced double-decker lutetium(III) phthalocyanines with charges of -1 to -5 and cationic sites of the 4C(8)P(+)Br(-) matrix. Both phthalocyanines undergo five one-electron reductions in the film, and a linear relationship was observed between the measured half-wave potentials (E-1/2) for these processes and the Cl- concentration of the electrolyte in the film. The apparent number of electrons transferred in each reduction step was obtained from plots of E-1/2 versus ln[Cl-], which gave Nernstian slopes of -23.5 +/- 2.3 mV (or -54.1 +/- 5.3 mV from plots of E-1/2 VS log[Cl-]) over a chloride concentration range of 0.005 to 1.0 M. The E-1/2 versus ln[Cl-] plots indicate that the stepwise addition of electrons to the phthalocyanines in the film is accompanied by a stepwise unit increase in the number of associated 4C(8)P(+) cations. Thus, the ratio of phthalocyanine molecules to monocations increases from 1: 1 for the singly reduced derivative to 1:5 in the case Of {[(C6H13S)(8)Pc](2)Lu}(5-) or {[(C12H25S)(4)Pc](2)Lu}(5-). This result leads to the conclusion that a binding of reduced [(C6H13S)(8)Pc](2)Lu or [(C12H25S)(4)PC](2)Lu to the cationic sites of the 4C(8)P(+) matrix is responsible for the experimentally observed positive shift of the measured half-wave potentials as compared to E-1/2 values for the same electrode reactions in nonaqueous media. The changes in E-1/2 upon changing the electrolyte counter cation and/or the charge of the matrix are also described, and these results further support the observed ion-pairing effects that were examined by means of electrochemical and spectroscopic techniques.