A new calcium ion selective and water-soluble calix[4]arene chromoionophore has been synthesised from the 25,27-bis[(ethoxycarbonyl)methoxyl-26,28-dihydroxy-calix[4]arene (1) and the N4,N4-diethyl-2-methyl-1,4-phenylenediamine-6-sulfonic acid (2). The 1,3-bis(indoaniline)-derived 2,4-bis-[(ethylcarbonyl)methoxy]-calix[4]arene (3) was obtained by introducing sulfonic groups into a phenylenediamine-derived chromophore followed by oxidative incorporation of two units of 2 into 1. The new chromophoric calix[4]arene 3 possesses an absorbance maximum at 604 nm in buffered aqueous environment. Upon complexation of calcium a bathochromic shift of 84 nm occurs along with an increase in the absorption coefficient. The dissociation constants and the cross sensitivity to other alkali and alkaline earth ions were evaluated. (C) 2000 Elsevier Science B.V. All rights reserved.