Solid solution samples Ag2xNa2(1-x)ZnSiO4 with 0 less than or equal to x less than or equal to 1 have been prepared by heat treating powder mixtures of Na2ZnSiO4 and Ag2ZnSiO4 at 300 degrees C and characterised by X-ray powder diffraction. The ionic conductivity of the solid solution has been determined as a function of composition and temperature by means of impedance measurements. The ionic conductivity of the mixed-mobile ion compositions (0 less than or equal to x less than or equal to 1) shows no substantial deviation from the mutually similar conductivities of the end compositions Na2ZnSiO4 and Ag2ZnSiO4, and neither activation energy nor conductivity pre-factor varies significantly with composition. The results indicate that the mixed-alkali effect on the ionic conductivity in cristobalite-related structures correlates with the size difference of the ions rather than with mass or polarisability differences.