Infrared absorption of the H2O stretching modes around 3400 cm(-1) is commonly used to detect the presence of H2O inside SiO2. Obtaining the exact concentration of water remains difficult since the molar absorption coefficient (epsilon) is not known. In this work epsilon is determined for H2O absorbed in sub-atmospheric chemical vapor deposited SiO2 and compared to liquid water. By integration over the whole absorption band width, epsilon is found to be 7.64 +/- 1.08 x 10(4) M-1 cm(-2). When considering peak absorption at the absorption band maximum only, the value is epsilon = 139.5 +/- 11.8 M-1 cm(-1) which is significantly lower than the literature value of liquid water for which epsilon = 231.66 +/- 0.05 M-1 cm(-1). This difference is attributed to the absence of hydrogen bridges between H2O molecules, evidenced by the featureless absorption spectrum of H2O inside SiO2. (c) 2013 Elsevier B.V. All rights reserved.