The radial diffusion coefficient for confined counter-passing MeV ions is evaluated in a new way, using measurements of escaping D-D fusion products. A class of passing ions near the plasma centre is measured whose orbits could eventually diffuse across their passing/trapped boundary, resulting in unconfined trapped orbits which can be detected at the plasma wall. The analysis indicates that the passing MeV ion diffusion coefficient D is smaller than ≈0.1 m2/s, which is small compared to the thermal diffusivities of ≈1 m2/s for these plasmas. The same experiment and analysis should be applicable to alpha particles in D-T plasmas.