We constructed velocity models of subducting slabs with a kinetically-depressed olivine →&bgr;- and &ggr;-spinel transition, and examined the effect that such structures would have on teleseismic P waveforms using a full-wave finite-difference method. These two-dimensional calculations yielded waveforms at a range of distances in the downdip direction. The slab models included a wedge-shaped, low-velocity metastable olivine tongue (MOTO) to a depth of 670 km, as well as a plausible thermal anomaly; one model further included a 10-m-thick fast layer on the surface of the slab. The principal effect of MOTO is to produce grazing reflections at wide angles off the phase boundary, generating a secondary arrival 0 to 4 seconds after the initial arrival depending on the take-off angle. The amplitude and timing of this feature vary with the lateral location of the seismic source within the slab cross-section. Careful analysis of waveforms from earthquakes with depths near 400 km, simple sources, and adequate station coverage in appropriate geometries will be required to resolve wheter MOTO is present. ¿ American Geophysical Union 1991