The repository contains spectral absorption and emission data [absorption cross section and Einstein coefficients] of rhodamine 6G dye solved in ethylene glycol at room temperature over the visible spectral range from 400.25nm to 619.85nm. In addition, typical values for the cavity loss rate are given for the same wavelength range. The data can be used e.g. for studies of two-dimensional thermalized photon gases and Bose-Einstein condensates of photons inside dye-filled optical microcavities. Methodology The absorption data has been obtained by white-light absorption spectroscopy of dye solutions with increasing concentration {0.01,0.1,1} mMol/Litre. The combined spectra have been calibrated with the rhodamine absorption cross section at 532nm wavelength, which we have independently determined in transmission measurements with a 532nm laser. The absorption cross section in this data repository constitutes a universal material property that is generally valid for rhodamine 6G solved in ethylene glycol at room temperature. The Einstein coefficient for absorption B_12 has been obtained specifically for the volume of the transverse ground mode in an optical microcavity formed by two curved mirrors with radius of curvature R = 1 and cavity length D = 1.5µm; see e.g. Klaers et al., Nature 468, 545–548 (2010), Schmitt, Phys. B: At. Mol. Opt. Phys. 51, 173001 (2018) and related work by the authors. For typical dye concentrations near 1mMol/Litre, approximately 10^8 molecules are contained in the ground mode volume. The Einstein coefficient for emission B_21 is deduced from B_12 assuming the Kennard Stepanov relation: B_21/B_12 = Exp[-h*c*(1/lambda - 1/lambda_zpl)/(k_B T)], where lambda_zpl = 545nm denotes the zero-phonon line of rhodamine 6G dye (h: Planck's constant, c: speed of light, lambda: wavelength, k_B: Boltzmann's constant, T: temperature). We have verified that the resulting B_21 spectrum agrees well with reference fluorescence spectra of rhodamine 6G. The spectral cavity loss rate c/(n0*D)*(1-R-A) with refractive index n0 = 1.43 and mirror absorption loss A = 1ppm is deduced from the wavelength-dependent mirror reflectivity R, which we have measured in cavity ring-down measurements at more than 10 wavelengths in the interval between 530nm to 605nm. For this, a tuneable dye laser was resonantly coupled into a 3.3cm-long cavity formed by the corresponding highly-reflecting dielectric mirrors. Note that the reciprocal values of the loss rates give the 1/e lifetime of the photons in the cavity. Data format The file 'data.dat' contains all data sorted by columns: wavelength (in units of nm), absorption cross section (in units of m^2), Einstein coefficients for absorption and emission (both in units of Hz), cavity loss rate (in units of Hz).