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This paper presents the results of the effects of electrode materials on tree initiation under switching surge conditions. Four kinds of electrode materials were used. Also, the effects of oxidized Al on the tree initiation was investigated. When a positive switching surge is applied to the needle in a point-to-plane test, little difference is found to exist among the four electrode materials with respect to the stress at the tip of the needle, Emax required to produce a tree. When a negative switching surge is applied, it is found that there are large differences of Emax. The value of Emax increases with the work function of the electrode materials. In addition with negative polarity, Emax increases with the rise time of the impulse wave in a manner which is related to the interaction of work function and space charge effects. It is noted that oxidized Al has a higher Emax than non-oxidized Al. When a positive half cycle wave follows a negative half cycle the value of Emax for the positive pulse is reduced from the value for a unipolar positive pulse by an amount which correlates with the work functio of the electrode.