The 'gabbro' model of the composition of the oceanic crust has prevailed for more than two decades, but serpentinites are common along much of the Mid-Atlantic Ridge, and elsewhere, leading to renewed debate over the abundance of serpentinites in crust created at slow-spreading ridges. Based on data from both oceanic and ophiolite samples, it has also been suggested that serpentinites are not distinguishable from meta-gabbros and diabases by their seismic properties. We have made new statistical analyses of the relationship between Vs and Vp in oceanic gabbro and diabase samples and serpentinized peridotites, and compared them to in situ s-wave and p-wave velocities derived from Expanding Spread Profiles (ESPs) acquired in the Atlantic. Our findings demonstrate: (1) that there is some overlap of the properties of gabbroic samples taken from ophiolites with the properties of serpentinites, but (2) over the range of typical-layer 3 seismic velocities, the seismic properties of oceanic gabbros and diabases are distinctly different from the properties of serpentinites, and (3) that in situ seismic velocities are in excellent agreement with the properties of the gabbros, while the shear velocities in serpentinites are significantly lower. Partially-serpentinized peridotite is certainly present, but not abundant in 'normal' oceanic crust.¿ 1997 American Geophysical Union