Dispersive Fourier transform spectroscopy in the visible (DFTS-VIS) is used to study the complex refractive index n of equilibrium mixtures of NO2/N2O4 in the gas phase between 290 and 350 K and in the wavenumber range between 11 000 and 19 000 cm(-1). With the aid of quantum-chemical MBPT/MP2 calculations of the components of the static dipole polarizability tenser of N2O4, the quasi-continuous spectra of the complex mean dipole polarizability alpha = alpha - i alpha’ of the neat components NO2 and N2O4 are obtained. On the other hand, for these neat components alpha(sigma(0)) at wavenumber sigma(0) = 15 798 cm(-1) is experimentally determined to be alpha(NO2)(sigma(0)) = 3,0233(37) x 10(-30) m(3) and alpha(N2O4)(sigma(0)) = 6.69(13) x 10(-30) m(3). The static limit of the mean dipole polarizability a of N2O4 is calculated to be a(0) = 6.43(19) x 10(-30) m(3). in addition, we report experimental results on the equilibrium constant K-p of the dimerization equilibrium N2O4 reversible arrow 2NO(2), obtained from the temperature and pressure dependence of n, as well as quantum-chemical calculations of the quadrupole and hexadecapole moment tensors of N2O4.