The progress of acetylation reactions of 4-aminophenol and 2-aminophenol in acetonitrile modified SF-CO2 or pure acetonitrile were monitored using on-line SFC or a non-conventional type of on-line normal phase liquid chromatography. For each reaction solvent, acetylations using acetic anhydride were performed at 323 K, 333 K and 343 K. Reactions in SF-CO2 were monitored using a single valve recirculation interface that coupled a high pressure reaction vessel operated at 17.5 MPa. Reactions in acetonitrile were monitored using a two valve interface to couple a reaction vessel operated under 0.1 MPa nitrogen atmosphere. For both aminophenol isomers, acetylation proceeded more rapidly in acetonitrile modified SF-CO2 compared with acetonitrile. Also for each reaction solvent, the rate of acetylation of 4-aminophenol exceeded that of 2-aminophenol. On-line chromatographic analysis rapidly established that: (0 4-aminophenol produced a diacetylated product only in acetonitrile modified SF-CO2, whereas (ii) 2-aminophenol produced a diacetylated product only in acetonitrile.