P-wave data from the short-period (around 1 Hz) Southern California Seismic Network reveal pre-critical reflections from the 410-km discontinuity below the Gulf of California, appearing as a clear and continuous feature in the data to epicentral distances as small as 10¿. Deconvolution and stacking techniques are used to extract these weak arrivals from the coda of earlier arriving P waves. The reflections are used to estimate the width of the gradient zone near 410 km depth, constraining the velocity jump across the discontinuity using critically refracted waves. The velocity jump across the 410-km discontinuity is 6¿2.5%, which is in agreement with existing estimates of the velocity jump for this region. The clear and uninterrupted branch of reflected waves suggests laterally homogeneous discontinuity properties on a scale of several hundred km. Synthetic seismogram modelling of the reflected waves using realistic models of the phase transition occurring at this depth shows that the transition must be as thin as 10 km, with most of the velocity increase occurring over about 4 km or less, for the reflections to be visible in short-period data. Applying this result to recent thermodynamic models of upper mantle composition suggests that the high temperatures associated with the spreading center continue down to at least 410 km depth. ¿ American Geophysical Union 1996.