Base-catalysed silica-based sols (composed of particles whose size increased from 11 nm to 90 nm as the water:TEOS ratio varied from 2.3 to 20) have been spin or dip deposited to produce highly porous anti-reflective low-density and high surface area coatings whose adsorption-desorption properties suggest that they respond or "breathe" reversibly to the prevailing atmosphere and in particular to its relative humidity.