Planarization of spin-coated polymer films is of great importance for the lithographic process in the microelectronic industry. Solvent retention is an important factor for determining the coating profile during spinning. In this research, solutions of polystyrene in toluene are spin-coated. The coating concentration during spinning is measured on-line using a laser interferometry technique. The optical measurement is evaluated by an off-line gel permeation chromatograph technique. The experiments indicate that a smooth coating surface is required to use the optical method for concentration measurement. The optical technique also provides information regarding the coating surface structure during spinning.