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In the present paper the pseudo Jahn-Teller interaction between the four lower electronic states of benzene is considered, with special reference to the mixing of the aBlu and 3Elu states through the e2g carbon-carbon stretching mode. First a review is given of the pioneering work of Liehr on the static problem, where the 'electronic Hamiltonian' is solved as a function of fixed nuclear coordinates. Results of a specific calculation are presented.Subsequently, the equations for the vibronic problem are set up by generalizing the methods proposed by Moffitt et al. and Longuet-Higgins et al. for the dynamic Jahn-Teller effect. These equations are then solved in the first (' pseudo-cylindrical ') approximation in which only the strong coupling linear in the displacements between the Blu and Elu states is considered. The results are discussed with reference to known properties of the phosphorescent state of benzene.