Changes in the electrical conductivity of carbon black filled EBA (ethylene butylacrylate copolymer) have been measured during thermal ageing and analysed. Above the composite melting temperature the conductivity increases steadily showing evidence of a relatively wide spectrum of relaxation times. This can be interpreted in terms of slow aggregation of carbon black clusters characterised by the ratio f of the effective volume fraction to the actual volume fraction of carbon black. The ratio f grows linearly with the logarithm of the time. The rate-limiting step in the process can be the carbon black particle - particle adherence that involves polymer displacement from the particle surface.