  Using partonic states we verify the transverse momentum dependent
factorization for single transverse spin asymmetries in semi inclusive DIS at
leading but nontrivial order of alpha_s. The factorization has been previously
derived in a formal way by using diagram expansion at hadron level. We find
with our partonic results that two relevant structure functions satisfy the
factorization. Our results also satisfy the collinear factorization but with
the perturbative coefficient different than that derived formally.
