Directed assembly of polymer blends using chemically heterogeneous patterns during spin-coating can be used to produce nanopatterned polymer structures. Well-ordered morphologies are obtained when the characteristic length of a polymer blend is commensurate with pat tern periodicity. In this paper, spin-coating speed and solution concentration were used to control the characteristic length of a polystyrene (PS)/poly(acrylic acid) (PAA) blend. With increasing spin speed or reducing solution concentration, the characteristic length decreased. Critical spin speeds or solution concentrations that produced the required characteristic length commensurate with the given pattern periodicity were predicted. Well-ordered morphologies were obtained when spin speed or solution concentration was close to the critical value. A new method of image analysis was introduced to quantitatively evaluate the quality of replication of the underlying pattern. The range of commensurability between characteristic length and pattern periodicity for well-ordered morphologies was investigated. When the range of commensurability was within 20%, well-ordered morphologies were produced.