Recycling of chemically stratified oceanic lithosphere at subduction zones and mantle mixing is the main source for the production and long-term maintenance of heterogeneities as evidenced through mantle samples and the presence of seismic scatterers. The mantle transition zone (MTZ), which is bound by seismic discontinuities around 410 and 660 km depth that reflect mantle mineral phase transformations in the olivine system, is considered to play a significant role in reprocessing and remixing of subducted oceanic plates. However, the compositional nature and mixing state of the MTZ is generally unconstrained with models ranging from a fully-equilibrated pyrolitic to a mechanically-mixed MTZ enriched in basalt. Here, we consider regional P- and S-waveforms from different regions that have interacted extensively with the MTZ and are triplicated at the seismic discontinuities. Because of the strong interaction with the MTZ and the sensitivity to mantle temperature and composition, triplicated waveforms can be used as probes of the thermochemical conditions in and around the MTZ. Here, we model the MTZ by considering two-endmember models: a fully-equilibrated and a mechanical mixture of depleted (harzburgite) and fertile (basalt) lithologies, whose properties are computed self-consistently using petrologic phase equilibria in combination with equation-of-state modeling as a function of pressure, temperature, and composition. Based on this method, we invert carefully selected triplicated P- and S-waveforms for local one-dimensional profiles of MTZ structure. In some of the studied regions, we find both radial and lateral heterogeneities in composition that show a trend from more fertile lithologies in the mid-MTZ to more pyrolitic compositions below the MTZ. This is in line with evidence provided by global geodynamic models that support the segregation and accumulation of basalt toward the mid-MTZ. Because a number of the regions studied here encompass subduction zone settings, we also observe lower-than-average mantle temperatures (∼1200∘C) and, as a consequence, thicker-than-average MTZ (∼240–280 km). While some areas appear to be better fit with a particular mixing model, rigorous statistical analysis of the datafit shows that on average, and at the level of the resolving power of the data, an equilibrated mantle across a large part of the studied subduction zone settings is favored.