The effects of a silicate melt and water on seismic velocities are compared at relatively small fluid fraction (less 20 vol.%) on the basis of a theoretical model of composite material, to show that the two fluids are clearly characterized by the velocity ratio Vp/Vs. For a silicate melt, Vp/Vs increases with increasing fluid fraction. When a significant reflection is expected, Vp/Vs becomes much larger than 2. For water, Vp/Vs decreases as the fluid fraction increases to 10 vol.% then it increases. But it remains similar to a solid state value (about 1.8), unless the amount of water exceeds 15 vol.%. This will be a good measure to distinguish between two candidates for seismic reflectors: partially molten rocks and rocks containing free water. If a reflector can be treated as a thin low velocity layer, the velocity ratio in it can be estimated from the frequency dependence of reflection coefficient. ¿American Geophysical Union 1993