Semi-inclusive asymmetries with polarized proton beams at HERA

The prospects of semi-inclusive measurements with polarized proton beams at HERA are discussed. Detailed simulations show that one can disentangle the valence-quark and sea-quark contribution to the polarized structure function g_1(x) in the small x-domain, if the equivalent of 1000 pb-1 of data are collected. It is also shown how semi-inclusive charged- current events can provide information on the relative importance of the spin contribution of the anti-s and anti-d sea quarks. Moreover, various methods to determine the fragmentation functions in this kinematical domain are presented.