Organic compound contamination in a clean room is presently attracting attention as a serious problem in F<sub>2</sub> laser lithography. Certain organic compounds may possibly have absorption spectra in the vacuum ultra violet region (VUV), as has been indicated by various theoretical considerations. But the effects of organic compounds present on a substrate surface are as yet unclarified. This study was conducted to determine whether organic compounds on the surface of a substrate serve significantly to reduce the transmittance of the substrate. The substrates of F-doped SiO<sub>2</sub> glass and MgF<sub>2</sub> were made to adsorb organic compounds by two different methods. Transmittance was measured at 130 to 200nm and clearly found to decrease as a result of organic compound contamination. In the case of dry-cleaned substrates, transmittance was recovered to its original level. It thus follows from these findings that substrate transmittance is significantly reduced by organic compound contamination on the substrate surface in the VUV region.