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The temperature- and frequency-dependent ionic conductivity in (Li1.xKx)2B4O7 glasses has been studied by using differential scanning calorimetry (DSC) and impedance/gain-phase analyzer. We have observed a mixed alkali effect from the anomalous change of the glass transition and crystallization temperatures. Dielectric properties in frequency and temperature domains have been analyzed by using the power law. We have found that there is a strong mixed alkali effect on the dc conductivity by observing the fact that the dc conductivity decreases by two orders of magnitude on mixing the alkali ions in the glass structure. The concentration dependence of the activation energy of the dc conductivity also shows a mixed alkali effect and the effect is understood with the structural random ion distribution model.j