We report on linear and third order nonlinear optical properties of the quinoid molecule 5,5'-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene)-5,5'-dihy dro-2, T-bithiophene. Variable angle spectroscopic ellipsometry has been used to determine the dispersion of the dielectric in a spectral range from the near infrared to the near ultraviolet. We have measured a high value of the index of refraction in the transparency region (i.e., n = 1.886 at lambda = 1.5 mum), indicating the high polarisability of the molecule under study. The nonlinear optical properties have been measured by third harmonic generation in the near infrared spectral range. At lambda = 1.5 muM the nonlinear coefficient of the molecule has been estimated to be 4.2 x 10(-11) m(2)/V-2 (3.0 x 10(-11) esu). (C) 2003 Elsevier B.V. All rights reserved.