A new method is introduced for the detection of chain interdigitation in phospholipid bilayers. The same method is used to measure the hydrocarbon tilt in the dipalmitoylphosphatidylcholine membranes as a function of the bulk concentration of the interdigitation-inducing solutes, such as ethanol. The hydrocarbon tilt in the phosphatidylcholine bilayers is demonstrated to be limited to angles below approx. 51 degrees. The need for higher tilt values leads to bilayer interdigitation. Solute-induced chain interdigitation is shown to be a cooperative process provoked by the excessively large lateral repulsion in the interfacial region and the concomitant excessive chain tilt. Ethanol-induced phosphatidylcholine interdigitation, for example, proceeds via interdigitated domains formation and finally gives rise to the bilayers with fully intercalated chains tilted by at least 30 degrees (and sometimes as much as 50 degrees) with respect to the membrane normal.