The boron nitride interphase and its interfaces in two-dimensional-SiC/BN/SiC composites have been analysed by transmission electron microscopy and electron energy loss spectroscopy. BN was deposited by isothermal/isobaric chemical vapour infiltration from BCl3-NH3-H-2 mixture at moderate temperature. BN and the fibre/BN interface exhibit different features depending on the nature of the Nicalon(TM) fibre surface, raw or treated prior to the BN deposition. When untreated fibres are used, a carbon-rich layer and silica clusters a re formed du ri ng the manufacturing of the composite. In that case, the interphase is poorly organized and presents a porous microstructure and a large carbon content. With the treated Nicalon(TM) fibre, no formation of a new. interlayer is observed at the fibre-BN interface and the interphase exhibits a better organized turbostratic microstructure with no voids. Additionally, in both types of composites, a carbon-rich layer is formed at the BN-matrix interface during the SiC infiltration step at about 1000 degrees C.