Attempts were made to connect the change in interfacial properties during fibre pull-out in cementitious material to the microstructural features of the interface. The microstructural features of fibre (steel, nylon and polypropylene)/mortar interfaces were examined during the fibre debonding and pull-out process. Because fibre pull-out was found to be sensitive to lateral compression, microscopic studies were carried out on fibres pulled out with and without lateral compression. SEM and energy-dispersive X-ray (EDX) analyses were performed at four different stages : (a) before debonding; (b) immediately after debonding; (c) at small sliding distance; and (d) at large sliding distance. For the steel fibre/mortar interface, it was found that the mortar surface (interfacial transition zone) was subjected to abrasion, while the steel surface was subjected to plastic deformation. EDX analysis on the mortar interface showed that the ratio of calcium/silicon count first increases within a short sliding distance and decreases thereafter, indicating a process of CH layer abrasion and C-S-H layer exposure. The rapid post-peak drop of the pull-out force at the beginning of sliding is due to the "grinding" effect, which leads to crushing and abrasion of the CH crystals and a reduction of asperity on the mortar surface. The grinding and abrasion effect becomes more significant with the application of lateral compression, which results in more rapid drop of the pullout force. For the nylon and polypropylene fibre/mortar interfaces, the fibre surface peels and the mortar surface experiences very little damage. Nylon fibre surface swells and is peeled with short whiskers on the surface, leading to significant increase in interfacial friction causing the post-debonding pull-out force to increase. The polypropylene fibre surface is peeled and plowed with long whiskers and long scratch lines which also leads to an increase in interfacial friction. On applying lateral compression to the mortar during fibre pull-out, the abrasion and peeling effects are more severe. With lateral compression, holes may form on the polypropylene surface over a longer sliding distance. The ratio of calcium/silicon count on the mortar surface by EDX does not show obvious trends with sliding distance indicating that the mortar surface experiences very little damage.