An in-depth study on the effect of process conditions (pressure, temperature and type of catalyst) on the acetylation of starch with acetic anhydride in pressurised carbon dioxide is described. A total of 22 experiments were performed and the experimental data were analysed using non-linear multivariable regression. The highest degree of substitution (DS) value (0.46) was obtained using K2CO3 as the catalyst at 90°C, 15 MPa pressure, and a catalyst to starch ratio of 0.5 mol/mol anhydroglucose units (AGUs). Important product properties of the acetylated starch prepared in CO2 like viscosity in water and relevant thermal properties were determined and compared with typical products prepared in an aqueous system.