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A study of the effect of absorbed water on the growth and configuration of electrical trees in composite insulation is described. Point-plane specimens were manufactured using clear polyester resin with a layer of reinforcing material cast midway between the electrodes. The trees were grown to demonstrate the response of composite materials to these trees under wet and dry conditions. The relationship between the barrier type and its resistance to tree growth when the composite had been subjected to water immersion was investigated. The mechanical strength of adhesion between the reinforcing material and the surrounding resin was measured for dry and wet specimens using standard mechanical tests. The effect of water absorption, both on tree growth and mechanical properties, was conducive to the water acting as a plasticising agent. To confirm this hypothesis comparative specimens containing 50% of a plasticising polyester were also studied and comparative observations made.Introduction.