The nu (2) + 3 nu (3) vibrational overtone of acetylene was recorded using an optothermal molecular beam spectrometer. The sample excitation region was surrounded by a build-up cavity and by electrodes which allowed the observation of Stark splittings of the spectral transitions. An analysis of the J dependence of the splittings determined that the (01300)(0) is mixed by the Stark field with the IR (infrared) dark state (11200)(0), and that the energy of the nu (1) + nu (2) + 2 nu (3) transition is 11 611.585 +/-0.018 cm(-1). The spectra also showed transitions to highly excited bending levels: These transitions were observable because they borrowed intensity from the more intense nu (2) + 3 nu (3) stretching overtone as they were tuned into coincidence. The bending states were found to have extremely high polarizabilities which may account for previously reported collision dynamics. (C) 2001 American Institute of Physics.