Abstract Ultrasmall CdSe semiconductor particles that exhibit size quantization effects are prepared in a perfluorosulfonate membrane (Nafion) by exposing Cd 2+-exchanged film to H 2Se. The growth of the particle can be controlled by varying the reaction time or the concentration of Cd 2+ in the Nafion film. CdSe particles exhibit an emission maximum in the region of 560–600 nm, the position of the maximum being dependent on the particle size. The emission decay is multi-exponential with lifetimes ranging into the nanosecond time regime. The photophysical behavior of CdSe particles in Nafion has also been probed with picosecond laser flash photolysis.