Laboratory studies established that corroding galvanized steel and hydrogen-producing bacteria can generate enough hydrogen to cause appreciable signal attenuation in optical fibers. Simulations of these two effects in the laboratory caused hydrogen indicators to increase up to 7.2 dB/km. The effect of hydrogen-producing bacteria can be suppressed by sulfate-reducing bacteria (SRB) in the absence or limited presence of fermentable hydrocarbons. In the abundant presence ofthese hydrocarbons, the optical fibers will attenuate, and even high concentrations of SRB cannot counteract this effect. During the fermentation process, acetic and butyric acids are produced. If these acids lower the pH of the environment below 5, the hydrogen is consumed by the formation of n-butanol and acetone. Data from fiber attenuation experiments in H2/N2 gas mixtures show that the attenuation at saturation increases approximately linearly with increasing p(H2) and decreases with increasing temperature.