A preference for acids When titanium dioxide surfaces are exposed to water under ambient conditions, an ordered overlayer forms. Balajka et al. studied this process with scanning tunneling microscopy and x-ray photoelectron spectroscopy for water adsorption under vacuum conditions and in air (see the Perspective by Park). The ordered overlayer was only formed in air, the result of the adsorption of organic acids (formic and acetic acids). Although other species such as alcohols were present in much higher concentrations in air, the bidentate adsorption and entropic effects favored acid adsorption. Science , this issue p. 786 ; see also p. 753