Energetic particle (0.1 to 100 MeV protons) acceleration is studied by using high resolution interplanetary magnetic field and plasma measurements at 1 AU (HEOS-2) and at ∼5 AU (Pioneer 10). Energy changes of a particle population are followed by computing test particle trajectories and the energy changes through the particle interaction with the time varying magnetic field. The results show that considerable particle acceleration takes place throughout the interplanetary medium, both in the corotating interaction regions (CIR) (5 AU), and in quiet regions (1 AU). Although shocks may contribute to acceleration we suggest statistical acceleration within the CIRs is sufficient to explain most energetic particle observations (e.g., McDonaldet al., 1975; Barnes and Simpson, 1976). The first and second order statistical acceleration coefficients which include transit time damping and Alfven resonance interactions, are found to be well represented byD T ≈8.5×10−6 T 0.5 MeV s−1 andD TT ≈4×10−6 T 1.5 MeV2 s−1 at 5 AU. By comparison, Fisk's estimates (1976), based on quasi-linear theory for transit-time damping, gaveD TT ≈5×10−7 T MeV2 s−1 at 1 AU.