A polymeric phosphine sensor is reported that exhibits bright blue fluorescence in the presence of gold(I/III) ions but is nonemissive with other metal ions. Specifically, solutions of a poly(p-arylenediethynylene phosphine) copolymer are 35 or 94 times more emissive when treated with solutions of (tht)AuCl or HAuCl4 center dot 3H(2)O, respectively. Model compound studies confirm phosphine coordination to metals, including gold(I/III) and rhodium(I), and the selective "turn-on" fluorescence was investigated using time-dependent density functional theory calculations.