The temperature-dependent hydration of several pure, net-neutral membranes was studied by spectroscopic shifts of the amphiphilic probe 6-dodecanoyl-2-(dimethylamino)-naphthalene (LAURDAN). A calibration scale for local polarity was first established with LAURDAN in various organic solvents. For phosphatidylcholine membranes above the gel-phase temperature, the log(local polarity) was found to be inversely related to the bending-renormalized area elastic moduli.-A novel self-assembled polymer membrane broadens the correlation, and the absence of any discontinuity in local polarity with temperature indicates that the polymer membrane is in a suitable fluid phase. Separate correlations between the log(hydraulic permeability x membrane thickness) and the bending modulus, but not the area elastic moduli, strongly suggest that, local bending fluctuations pf a membrane are coupled to membrane hydration. The results identify the importance of a collective response, membrane flexure, to a molecular-scale property, specifically, hydration.