We present the on-surface reduction of acene-derivatives to form genuine acenes (hydrocarbons with linearly fused benzene rings) on Au(111) and Cu(111). The conversion is achieved by scanning tunneling microscopy (STM) tip-induced manipulation as well as thermal activation and is conclusively demonstrated by means of atomic force microscopy with atomic resolution. This observed transformation may pave the way for the on-surface preparation and stabilization of even higher acenes.