Development of selective optical chemosensory systems for the determination of metal ions is very important fields in supramolecular chemistry. Particularly interesting is the design of fluorescent sensors which can selectively detect biologically important metal ions, such as Na+, K+, Mg2+, and Ca2+,1,2 We have synthesized a new chemosensor having phenanthroimidazole subunit3 based upon the calix[4]arene-diamide and investigated its Mg2+ and Ca2+-selective fluoroionephoric properties. The compound exhibited pronounced Mg2+ and Ca2+ selective fluoroionophoric properties over other alkali, alkaline earth, and transition metal ions. Optimization of the Mg2+ and Ca2+-selectivity was attempted by using a series of aqueous DMSO solutions. Significant red shift in fluorescence emission (Äë = 84 nm) upon interaction with Mg2+ and Ca2+ ions suggests the naked-eye detection and also allowed the possibility of ratiometric determination of Mg2+ ions in the presence of other physiologically important alkali and alkaline earth metal ions. The detection limit for the Mg2+ determination was found to be 4.2 x 10-6 M, which suggests the possible application of the compound in the analysis of Mg2+ ions in pseudo-physiological environments. The details of the optimization of the chemosensor toward Mg2+ and Ca2+ ions in terms of solution composition and other variables such as solution pH will be discussed.