Seismic wave attenuation (1/Q) is a physical property that might be considered in seismic tomography to improve the subsurface imaging. In particular, it may help in the exploration of unconventional reservoirs as these resources are hosted in highly attenuating geo-materials. One of the factors increasing visco-elasticity of rocks (i.e. 1/Q) is the presence of fluids. Here we report experimental data showing how gas bubbles, occupying part of water-saturated pores, attenuate seismic waves. The data are explained with a gas-exsolution-dissolution theory and a 1D numerical model accounting for the diffusion of the gas in the water. The theory is then applied to an oil-methane system showing that this attenuation mechanism is relevant also for hydrocarbon reservoirs.