It has been demonstrated that - in certain cases - the sizing of carbon fibres can have a dramatic effect upon the mode of failure of unidirectional fibre-reinforced composites. The sizing appears to reduce the strength of composite tows by confining the failure process to a very small area that exhibits high stress concentration. In this paper, the effect of fibre sizing upon the two-dimensional fibre break density and break cluster populations is investigated as a function of applied strain prior to composite failure. It is shown that the size of the damage sites, their spatial distribution in the composite and the alignment between the individual breaks in the cluster are affected by the interface properties. Fractographic analysis has shown that groups of adjacent fibre fractures of greater than three were observed for the sized composite tows, whereas for the unsized samples a higher proportion of single and double breaks were seen to exist at a particular stress level. As a result, the overall filament damage was seen to be more widespread in the case of the unsized composite tows. Two possible mechanisms of fracture nucleation based on changes in fibre break density and in cluster populations are proposed: (a) failure due to growth of a critical cluster of fibre fractures and/or (b) linking up of several smaller cluster to form a critical cluster.