During the flyby of the Giotto S/C with comet Grigg-Skjellerup (G-S) (9–11 July 1992) energetic particles (EH2O > 60 keV, Ep > 30 keV, Eα > 3.5 MeV), magnetic fields and plasma ions were measured by three different detectors on board Giotto. The flyby period was characterized by high magnetic field magnitudes (> 12 nt) and an enhanced background flux caused most likely by the reverse shock of a corotating interaction region (CIR). Within the CIR three different particle acceleration events were detected: 1. Proton and α-particle acceleration (Ep > 4.5 MeV, Eα > 3.5 MeV) caused by a sudden increase of the magnetic field combined with a change in direction. 2. Proton and H2O-ion (Ep > 220 eV, EH2O > 260 keV) acceleration caused by a directional discontinuity of the magnetic field. 3. Acceleration of water group ions in the foreshock and bow shock regions of comet G-S associated with a further increase of the magnetic field magnitude. Particle fluxes, magnetic field measurements, pitch angles and energy spectra are used to discuss the particle acceleration processes such as shock drift mechanism, inductive acceleration, Fermi acceleration and adiabatic compression. The results are compared with those obtained during the Giotto-Halley encounter.