Phase change materials (PCM) are capable of storing thermal energy within a small temperature range due to their high latent heat. In simulations of thermal energy storage (TES) systems with PCMs, natural convection is often not considered, possibly resulting in differences between numerical and experimental results. Since natural convection depends on the viscosity of the melting or melted region, it is important to determine this property in PCMs accurately. This paper presents different measurements carried out with octadecane. Both rotational and oscillatory tests have been carried out and the influence of the gap and the heating and cooling rate on the results has been analyzed. A first approach to the measurement methodology for obtaining the viscosity of octadecane during phase transition and melting is proposed, using a plate as geometry and a Peltier plate as temperature controller. (C) 2012 Elsevier B.V. All rights reserved.