It is demonstrated theoretically how the ionic association of the metal cation and/or protonated form ofthe neutral indicator (chromoionophore) with the anionic sites in the polymeric optode phase alters thedynamic range and the sensitivity of the optode response. Explicit response equations are derived forseveral limiting cases where the metal ion or the protonated indicator form ion pairs with the anionicsites. The optode response is also simulated numerically with no assumptions on the association degreeof any type of the species present in the membrane phase. Simulations show that association in theoptode phase may result in significant errors when optode is used as an analytical tool. The results areconfirmed experimentally using pH/pM optodes based on chromoionophore ETH2439 as a model system.It is demonstrated that by selection of a proper ionic additive one can optimize the optode response rangefor the desired concentration range.