At Cascadia accretionary prism, log-based porosities exhibit complex lateral changes in response to variations in cementation, horizontal compression, and fracturing. Porosities at the prism toe are lower than in reference sediments, whereas porosities landward of the toe are slightly higher than those of reference sediments. Ideally, a porosity/velocity relationship can be used to determine two-dimensional porosity structure from seismic velocities. Porosity is the major control on velocity at Cascadia, but variations in grain framework strength (frame bulk modulus) cause deformed and undeformed rocks to have quite different velocity/porosity relationships. Within the prism, cementation can raise velocities substantially above those for unconsolidated reference sediments of the same porosity; also, compression-induced compaction and fracturing may affect this velocity/porosity transform. Because these factors vary both within and between prisms, velocity/porosity transforms are local, not universal.¿ 1997 American Geophysical Union