This work gives a comprehensive view of the surface acylation of cellulose nanocrystals (CNCs), as a tool to monitor Pickering emulsions. It investigates the impact of the grafted chain length and surface degree of sub- stitution (DSsurf), on the type (direct or inverse) of such emulsions. CNC samples were prepared by grafting 10 different linear acyl groups containing 2 to 18 carbons, with reaction times from 30 min to 5 h to vary the DSsurf. The grafting was evaluated by FT-IR and 13C CP-MAS NMR spectroscopies. Whatever the DSSurf, CNCs grafted with linear acyl chains of 2 to 6 carbons led to the exclusive formation of direct oil-in-water (O/W) emulsions with hexadecane. Distinctively, both O/W and water-in-oil (W/O) emulsions could be obtained when the linear chain contained 8 carbons or more, at low and high DSSurf, respectively. By adjusting the length of the grafted chain, DSSurf and particles concentration, we were able to monitor the type of emulsion formed, droplet size and surface coverage at the oil/water interface.