  The physics of polarized nucleon--nucleon collisions originating from an
internal polarized target in the HERA proton beam is investigated. Based on 240
pb^-1 integrated luminosity at 40 GeV c.m. energy, statistical sensitivities
are given over a wide (x_F, p_T) range for a variety of inclusive and exclusive
final states. By measuring single spin asymmetries unique information can be
obtained on higher twist contributions and their p_T-dependence. From double
spin asymmetries in both photon and J/psi production it appears possible to
measure the polarized gluon distribution in the range 0.1<x_gluon<0.4 with a
good statistical accuracy.
