This paper presents a numerical simulation of the shock wave refraction over an interface of two different gases. Harten-Yee upwind TVD scheme is used and an approximate Riemann solver has been developed specially for this problem in which the interface of the two gases can be treated without any peculiar consideration. Some experimental data have been obtained for the refraction of a planar shock at the air/CO 2 and air/He interfaces. The refraction and reflection of the shock wave as well as the flow field around it are visualized by using double exposure holographic interferometry. The numerical simulation results were compared with the experimental results, good agreement was obtained. Based on the one-dimensional piston theory, the conditions for both BPR (Irregular Refraction of the Bound Precursors) into FPR (Free Precursor Refraction) and FNR (Free Precursor von Neumann Irregular Refraction) into TNR (Twin von Neumann Refraction) transitions are analysed. The crater of the gaseous interface near the boundary is also discussed in this work.