These comparisons are a clear indication that the gas-phase protocol has better regioselectivity, especially at higher temperature, compared to the liquid-phase methods. This, combined with the low chemical demand, ease of product recovery and benign nature of gas-phase reactions, make it highly suitable for industrial implementation. Finally, we decided to assess the regioselectivity, and provide broad insights on acetylation kinetics, for a series of different cellulose substrates (Figure ): 1) FD-CNCs, 2) spray-dried CNCs (SD-CNCs), 3) CNC Aerogel, 4) IONCELL fibres (regenerated cellulose II air-gap spun fibres, using an ionic liquid direct-dissolution solvent) , 5) pre-hydrolysis hardwood (mainly birch) kraft pulp (P-H kraft Pulp), 6) beech sulphite pulp. All samples were analysed after reaction at RT for 6 d. DS and crystallinity data are given in the supporting info (Table ). to the FD-CNCs. It is expected that spray-drying results in much reduced surface area, through aggregation, compared to freeze-drying. XPS analysis (SI Figure ) of the samples also showed the presence of silicates which is expected to occupy reactive sites and potentially facilitate aggregation during spray-drying. Similarly, the CNC Aerogel sample is expected to maintain a high surface area. Surprisingly, this resulted in a higher DS and high regioselectivity, compared to the FD-CNCs. Naturally, the BH-PHK and BH-S pulps should have rather low surface areas, due to their much more complex morphology and hornification during the drying procedures. This is reflected in their lower DS values, which however do seem to maintain high regioselectivity. Finally, the IONCELL reaction also gave similar DS values to the pulp samples, indicating a large degree of aggregation and low surface area after drying, preventing further reaction under ambient conditions. In the case of the patent concerning gas-phase acetylation of regenerated cellulose fibres , much higher degrees of acetylation were possible (clearly indicating non-selective CTA formation), but only after pre-swelling with potassium acetate solution. Thus, specific surface area and porosity of dried materials are of clear importance, for controlling regioselectivity yet reaching high degrees of reactivity. The low reactivity of the fibrillar cellulose samples will likely also be alleviated to some extent using higher temperatures. Initial indications in this study suggest that the regioselectivity may suffer with increasing temperature. However, this can be minimised by targeting lower DS values, as full crystallite surface coverage is likely not required in all cases. This may also be critical in some cases to prevent exfoliation of chains, under solvating conditions.