Legal Question in Landlord & Tenant Law in Illinois

Depends on whose perspective. Why would you do this without your co-tenant's ok? When 2 people rent an apartment together it's like an unwritten partnership. Normally in a partnership either partner can "bind" the partnership to an arrangement with a third party, so it is possible. But if the groundrules between the two of you is that neither of you does certain things without the other's ok, then no. If it's a question of whether you have the right under the lease, however, that's a different story. In Chicago (based on your zip code) if your building meets the requirements of the Chicago Residential Landlord & Tenant Ordinance, your lease should allow you to sublet on reasonable terms.