Solvent concentration is an important factor for spin-coated film planarization. Two techniques, one an on-line optical method and the other an off-line gel permeation chromatography method, have been used to investigate the coating concentration change during spinning. The experimental observations are compared with the spin-coating model predictions. Tile modeled concentration and thickness are in close agreement with experiment. Under certain conditions, the spin-coating model can be significantly simplified. These conditions are discussed.