OCT has emerged in recent years to a powerful technique to measure tissue properties. Recently, OCT has been enhanced by methods providing spectroscopic information. Since water is a major constituent of most tissues, the measurement of its concentration in tissue is important for tissue diagnostics. We present measurements of water absorption in human cornea in vitro with a differential absorption optical coherence tomography technique. This technique uses two OCT images recorded simultaneously with two different light sources, one centered within (1488nm) and one centered outside (1312nm) of a water absorption band. To study influences of scattering on the absorption images and on the calculated differential absorption coefficient, the cornea was measured and imaged under three different conditions: At first, the cornea was imaged in a hydrated condition, immediately after removal from the aqueous nutrient solution. Then it was dehydrated and imaged a second time, finally it was rehydrated, however, the water was replaced by Deuterium oxide, which shows negligible absorption in the used wavelength region, but otherwise has similar optical properties as water. The cornea containing H2O is well distinguishable from the cornea containing D2O with our method. For quantitative determination of absorption, we performed a linear regression analysis of logarithmic backscattered intensity versus imaging depth in the cornea for each wavelength. The difference of the slopes corresponds to the difference in the absorption coefficient. If the difference in the water absorption cross section is known, the water concentration in tissue can be calculated. The results are in good agreement with those expected theoretically.