The conversion from fast to slow positrons by several thin (0.3–7 &mgr;m) film moderators has been studied. Positrons from a radioactive22Na source enter the film where some are thermalized, consequently diffuse to the surface opposite to the source and are emitted with low energies (1–4 eV). The experimental data show that the conversion efficiency to slow positrons ranges from 1×10−4to 9.1×10−4for good moderators. The moderator efficiency was found to be very sensitive to annealing temperatures.