We present observations of 130- to 1200-keV protons and 40- to 420-keV/nucleon alpha particles made on the earth-orbiting spacecraft Imp 8 and Imp 7 during an active solar period in September 1974, concentrating in particular on an energetic storm particle (ESP) event observed in association with an interplanetary shock wave on September 21. We find that the observed variations in the proton to alpha particle ratios and spectral indices can be explaned either by 'pileup' or by acceleration models of ESP events. We also discuss several instances of local acceleration of particles in the near-earth environment.