Retention data are presented for homovanillic acid, vanillic acid, phenylacetic acid, benzoic acid, p-hydroxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid on reversed phase C18columns with methanolic-acetic acid buffers. These data are fitted to a model and window diagrams are constructed from calculated model parameters. The different window diagrams obtained for the different percentage methanol solvents are shown. The retention data is then fit to a two factor reponse surface model (% methanol and pH).