Carbon fibre surface treatments can affect the composite properties in the fibre direction. Two effects play an important role in the longitudinal tensile strength. First, the carbon fibre surface treatment has some influence on the individual fibre strength. Second, an improved interface strength leads to better load transfer in the composite. Using an elastic-slip model, it is shown that the ineffective length increases dramatically with decreasing degree of surface treatment. It is also shown that, as the debonded length of a broken fibre decreases, the strain in the matrix becomes very high, leading to matrix cracking and additional stress concentrations on the neighbouring fibres. This leads to a maximum strength at intermediate treatment levels. This conclusion is supported by UD tensile tests on 0 carbon/epoxy laminates.