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The radio frequency safety of mobile phones has been evaluated in terms of specific absorption rate (SAR). Standard methods for measurement of the SAR, including recipes for tissue-equivalent dielectric liquids, have recently been the subject of discussion among international standards organizations. Standards currently recommend glycol-type liquids as tissue-equivalent liquids for frequencies above 1 GHz. Although the ingredients are specified in the recipes provided, some fundamental information, such as the stability of dielectric properties, remains unclear. We measured the change of dielectric properties with time and with temperature of tissue-equivalent liquids recommended in the standard documents, and evaluated their effects on SAR. The conductivity decreased with increasing temperature in all glycol-type specimens. The permittivity, on the other hand, was almost constant. With the evaporation of water, the permittivity decreased, although the conductivities remained constant. Experimental results proved that dielectric properties are affected by environmental conditions, and that it is inevitably necessary to adjust the dielectric properties regularly, through the addition of ingredients, in order to follow the standards. The SAR values, however, were not affected significantly by the change in dielectric properties; thus, a larger tolerance of the dielectric properties may be acceptable in practical SAR measurements.