A novel validation experiment for the melting of a phase change material is presented. The goal is to measure phase state and velocities with high accuracy and resolution. The geometry and boundary conditions of the test section are the most generic found in latent heat storage systems: a phase change material is contained in a rectangular enclosure, where it is isothermally heated from two opposing vertical side walls. The enclosure has a height of 105 mm and a width and depth of 50 mm. The bottom, front and back sides are solid transparent walls and on top is a thin layer of air. Near-adiabatic boundary conditions are realized at the non-heated sides with a surrounding insulated air-filled chamber and an actively controlled trace heating system. In this study n-octadecane is used as the phase change material. During melting, the liquid phase fraction is measured with a shadowgraph technique and velocities due to natural convection in the liquid phase are measured with particle image velocimetry (PIV). Interior and boundary temperatures are measured with thermocouples to control and analyze boundary effects. A thorough error estimation is done for all the measured quantities. The main result is a comprehensive dataset of liquid phase fractions and velocities with high spatial and temporal resolutions. The liquid phase fraction is additionally measured for three different driving temperature differences and a scaling by dimensionless numbers is performed. This results in a correlation function for the liquid phase fraction that predicts similar melting processes and is valuable in system design and optimization. (C) 2018 Elsevier Ltd. All rights reserved.