Phase change materials (PCM) used in latent heat storage systems usually have very low thermal conductivities. This is a major drawback in maintaining the required heat exchange rate between PCM and heat transfer fluid. This paper investigates the enhancement of the heat transfer between PCM and heat transfer fluid, using high thermal conductivity as additives like stainless steel pieces, copper pieces and graphite-PCM composite material. In the experiments, palmitic-lauric acid (80:20) (PL) and stearic-myristic acid (80:20) (SM) were used as PCMs. Test results show that heat transfer enhancement of copper pieces was better at 0.05 L s(-1) flow rate compared to 0.025 L s(-1). Using copper as an additive increased the heat transfer rate 1.7 times for melting and 3.8 times for freezing when flow rate was 0.050 L s(-1). Decreasing the flow rate from 0.050 to 0.025 L s(-1), increased the melting times 1.3 times and freezing times 1.8 times, decreasing heat transfer rates accordingly. The best result of heat transfer enhancement was observed for the PCM graphite composite. However, changing the flow rate did not affect the heat transfer rate when graphite was used as additive. Copyright (C) 2007 John Wiley & Sons, Ltd.