We have synthesized and spectrally characterized a squaraine-based fluorescent chemosensor for pH. This chemosensor unlike many others, works in 100 % aqueous solutions, signalling the pH change from 10 to 7 by a 11-fold increase in the emission intensity at 651 nm, when excited at the isosbestic point (614 nm). The average pK(a) for the ionizable groups is found to be 8.8. The chemosensor with its molar extinction coefficient > 200,000 and the quantum yield of 0.2, yield an impressive intrinsic brightness value of 40,000. (C) 1997 Elsevier Science Ltd.