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Abstract: The present study concerns the system 0.5[xNa2O–(1− x)Li2O]–0.5[0.4(MoO3)2–0.6P2O5], with x varying between 0 and 1. The elaborated glass samples have been characterised using powder X-ray diffraction (XRD), differential thermal analysis (DTA) and impedance spectroscopy. The total alkali oxide content is chosen in order to ensure that the electrical conductivity of the glasses under study is entirely ionic. It was evidenced that the conductivity minimum corresponds to the maximum of the activation energy. The glass transition temperature seems to present a minimum for the ratio Na/(Na+Li)=0.5. On the other hand, the density appears very linearly versus Na/(Na+Li). The mixed alkali effect (MAE) is resulting here in a conductivity minimum observed for the composition corresponding to Na/(Na+Li)=0.5, (x =0.5). It could be attributed to the fact that the two types of alkali ions are randomly mixed and have distinct conduction pathways. This implies that Li ions tend to block the pathways for the Na ions and vice versa. [Copyright &y& Elsevier]