Scatter-free acceleration of energetic protons by quasi-perpendicular interplanetary shocks is investigated. Pitch angle distributions of 35--384 keV protons have been determined upstream of four interplanetary shocks that passed through the ISEE-3 spacecraft during 1978--1979. Bidirectional, loss-cone-type angular distribution of particles observed at events with the closest to perpendicular field line to shock normal angle suggests that the magnetic field line may have crossed the shock front several times, forming traps on the upstream side. The model was tested by numerical trajectory calculations, resulting in a qualitatively similar angular distribution to those observed. The loss cones, appearing in the observed as well as in the simulated pitch angle distributions imply that the first adiabatic invariant (magnetic moment) is at least approximately conserved during the shock interaction process. The violation of magnetic moment conservation was investigated in more detail by simulating the trajectories of particles in various trap geometries. It was found that the invariance is fairly well satisfied when the magnetic field line grazes the shock surface. The conservation of the second (longitudinal) adiabatic invariant was also studied numerically. A possible role which the longitudinal adiabatic invariant may play in the control of the particle release from the trap is pointed out. It is suggested that the fluctuations in the energetic particle flux, observed frequently on the downstream side, could be explained by the nonuniform speed of trap collapse. ¿ American Geophysical Union 1993