In recent years, the use of natural fibres as reinforcement in polymer composites has attracted increasing attention for partial or complete replacement of conventional inorganic fibres. Besides their lightweight, plant fibres have the advantage of low cost, prevent tool wear during processing and show damage-tolerant material behaviour. Cellulose fibres exhibit a highly polar surface due to the presence of hydroxyl groups. These hydroxyl groups enable the formation of hydrogen bonds in the interface region of composite materials. However, in order to get access to these hydroxyl groups, a cover of pectin and other waxy substances must be removed from the fibre surface. On the other hand, the high polarity of the cellulose fibre surface is the reason for their hydrophilic behaviour, which induces fibre swelling due to moisture uptake. Intense surface treatment is, therefore, required for fibre cleaning and preparation for interfacial bonding. In the case of natural fibres the use of coupling agents like silanes serves not only as an adhesion promoter but also to hydrophobize the fibre surface and to prevent swelling processes. Such surface treatment may be monitored and controlled by the electrokinetic method of streaming potential. Results of streaming potential measurements for the characterization of fibre swelling and for the elucidation of the interaction between fibres and coupling chemicals are presented. Solvatochromism results are used to confirm the electrokinetic measurements.