Abstract The erbium–hydrogen–zirconium (Er–H–Zr) ternary system has been investigated experimentally at 350 °C using high purity materials. The extent of the ternary homogeneity domains have been measured using two combined experimental techniques: X-ray diffraction and ion beam analysis. An isothermal section is proposed according to these measurements. The three binary systems Er–H, Er–Zr and H–Zr were already assessed using the Calphad method. After making them compatible with each other and assigning the enthalpy of formation of the new generated end-members (including in H–Zr system) to the results of DFT calculations, the ternary system has been calculated without using any ternary parameter. The calculated isothermal section at 350 °C shows a fair agreement with the experimental data and the behavior at higher temperature is predicted.