Electronic excitation in thin polymer films by MeV ion beams and on-line transient mass spectroscopy together provide a new opportunity for measuring the diffusion of molecules in polymers even when the diffusion coefficients are <∼ 10−10 cm2 s−1.With a quadrupole mass spectrometer, we have obtained transients of several molecular species from a variety of polymers bombarded by 2 MeV helium and argon ions.We have measured, for example, transient emission of D2, CO, CO 2, CD3OD, etc., from deuterated Poly(methyl methacrylate), PMMA.The beam fluences required for the measurements are sufficiently small that they cause minor perturbation in the structure of the film. The exciting ion pulse is typically 15 seconds wide with fast (<msec) rise and fall times. However, the emission signal of m−46 (COOD), for example, shows a slow build-up with a gradual fall after the beam is turned off with a tail lasting several seconds. The experimental data for the above molecule has been fitted to a one-dimensional diffusion model and the diffusion coefficient deduced from three different film thicknesses is ∼8−±2×10−11 cm2S−1 Since very thin films can be used in these experiments, still smaller D values are measurable. The sample requirements are simple and even the presence of pinholes will not affect the measurements significantly. Techniques that completely avoid any possible effects of ion damage tracks and an example where chemical reaction is the rate limiting step will be discussed.