Optical second harmonic generation (SHG) at the liquid-liquid interfaces of ionophore-free and neutral ionophore-incorporated membranes has been measured in the presence and absence of ionic sites in order to better understand the influence of ionic sites on the potentiometric responses of liquid membrane ion-selective electrodes. Quick and lasting EMF and SHG responses were observed upon increasing the primary ion activity in aqueous solutions in contact with membranes with ionic sites. For membranes without ionic sites, transient EMF responses due to extraction of primary ion salts occurred. They were accompanied by very similar transient changes in the SHG intensity. No EMF and SHG responses to the primary ion activity were, however, observed for site-free membranes in equilibrium with the aqueous phase. The extent of charge separation at the interfaces of such membranes, as observed by SHG from primary ions or primary ion complexes, does not depend on the primary ion activity in the aqueous phase. This explains why membranes free of ionic sites do not exhibit EMF responses and confirms that ionic sites are a necessity for counterion-independent primary ion responses of potentiometric sensors based on neutral carriers.