Two-photon spectroscopy is used to measure the frequency of the Kekule vibrational frequency of several benzene derivatives in the first excited state (SI). It is found that the frequency is always increased with respect to the ground state, and that it is essentially constant. This finding is in line with a model presented by the authors, in which the ground and first excited states of benzene are treated as 'twin states'. The physical nature of this vibration is discussed in view of quantum chemical calculations.