This paper reports the synthesis, optical, and thin-film-forming properties of a new water-soluble poly(phenylene ethynylene) that features phosphonate solubilizing groups. The new polymer, PPE-PO3-, is prepared by a synthetic route that involves initial preparation of a neutral analogue polymer in which the phosphonate groups are in the form of dibutyl esters. The neutral polymer is converted to the water-soluble form by cleavage of the phosphonate ester groups using (CH3)3SiBr. PPE-PO3- absorbs in the blue of the visible region and features a moderately intense fluorescence. The absorption and fluorescence of the polymer vary strongly with pH, an effect which is believed to arise because the polymer exists in a strongly aggregated form at low pH and in a relatively unaggregated state at high pH. Ultrathin multilayer films of the PPE-PO3- are deposited by the layer-by-layer (LbL) method using either the cationic polyelectrolyte poly(diallyldimethylamine) or Zr(IV) as deposition “partners”. Absorption spe...