A fluorescent film sensor was prepared by chemical assembly of pyrene on a glass plate surface via a long flexible spacer. It was found that the film is highly selective for some organic Cu2+ salts, such as copper acetate and copper propionate. The presence of inorganic Cu2+ salts and other metal(II) acetates, including Ni2+, Co2+, Pb2+, Cd2+, Zn2+, etc., had little effect upon the sensing behavior of the film for copper acetate or copper propionate. The observation was explained by employing a proposed "two-dimensional solution" model. The quenching by copper acetate of the emission of the film is static in nature due to complexation of the spacers to the metal ions. Furthermore, the response of the film sensor to copper acetate is fully reversible. To the best of our knowledge, this film sensor may be the first one that can differentiate greasy copper salts from inorganic copper salts.