We have developed a fluorescence lifetime based energy transfer sensor for detecting copper(II) ions. Rhodamine 800 in Nafion selectively transfers its energy to copper ions when excited at 670 nm. By fitting to the fluorescence decay we can resolve copper concentrations in water down to the level of 10 ppb. Time-correlated single-photon counting is used for detection. Good discrimination against interference by cobalt, nickel and chromium ions is obtained by virtue of the reduced spectral overlap of these ions as compared to copper. Possibilities for an energy transfer sensor for both copper(I) and copper(II) ions are discussed. Application of the theory of resonance energy transfer is investigated for use in sensor matrices using a range of donors and acceptors and the appropriateness of current models discussed.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.