Most empirical equations express the thermal conductivity of normal simple liquids as linear functions of temperature. The majority of measurements to date, except water, support such a description. Very precise new measurements on toluene over the complete liquid range at 1 atm (0.299 < Tr < 0.645) show a substantial pre-freezing anomaly. At the freezing point (tf) there is a depression of the expected conductivity of 7.9 × 10−3 W/m °K. This tf behavior is attributed to cluster formation.