Fluorimetric and Computer Screen Photoassisted Technique (CSPT) studies on the binding properties of three coumarin pendant arm derivatives of mixed aza-thioether macrocyclic ligands (L1-L3), incorporated into PVC plasticized polymeric membranes, towards a number of different metal cations were carried out. An enhanced selectivity of the ligands towards Hg2+ ion was detected. In particular, the DOS plasticized membrane containing L3 and tetrakis(p-chlorophenyl)borate, TpClPB-, as lipophilic anionic additive, displayed a strong fluorescence activation by Hg2+ over the wide concentration range of 10-7 to 10-3M. The influence of the ratio among ligand and lipophilic additive on the membrane performance was tested. In addition, the CSPT measurements showed that L3 possesses a unique selectivity towards Hg2+ ion with respect to several common diverse cations. Moreover CSPT technique permits to perform the fast and low cost monitoring of mercury (II) ions, by application of familiar devices and a chemometric approach