We report first non-perturbing measurements of energy distribution function ƒα (Eα) of confined DT fusion alpha-particles, made in the Joint European Torus (JET) using a neutral particle analyzer. We demonstrate significant direct neutralization of α-particles in charge-exchange reactions with helium-like ions of main intrinsic plasma impurities in high temperature plasmas. We have also measured for the first time in a tokamak plasma MeV energy deuterons produced by close collisions between fusion alpha-particles and plasma fuel ions, which are neutralized by hydrogen-like ions of the impurities. Both processes contribute to measured flux to the analyzer and allow ƒα (Eα) to be determined.