The amphiphilic chromo-ionophore 2-[(1E,3E)-4-(2,3,5,6,8,9,11,12,14,15-decahydro-1,7,10,16,4,13-benzotetr aoxadithiacyclooctadecin-18-yl)-1,3-butadienyl]-3-octadecyl-1,3-benzothi azol-3-ium perchlorate (AB211) forms mixed monolayers at the air-water interface with the phospholipid L-alpha-dimyristoylphosphatidylcholine (DMPC) as deduced from surface pressure-area and surface potential-area isotherms. In contrast to the behaviour of pure monolayers of AB211 on water, no association of the chromophores is observed by reflection spectroscopy. Upon compression of the mixed monolayer AB211:DMPC = 1:5, a reorientation of the chromophores from flat to tilted is observed, presumably assisted by the stretching of the choline group of the phospholipid molecules acting a matrix. (c) 2004 Elsevier B.V. All rights reserved.