During the solution polymerization of n-butyl acrylate in dioxane, the monomer concentration was monitored by on-line Raman spectroscopy. It was shown that it is quite possible to control the monomer to solvent ratio by a feedback monomer addition strategy, when the spectroscopic data is analyzed in real-time. A semi-continuous polymerization which was purposely subjected to disturbances, was controlled without abrupt changes in the monomer to solvent ratio and the monomer flow rate. Further, it was shown that controlling the process leads to more narrow molecular weight distributions.