Experiments are presented that explore the shock initiating layer dynamics in an insensitive high explosive. Tests were conducted with a PBX 9502 slab bonded on one side to a PBX 9501 slab. For each test, a detonation in the PBX 9501 was generated to drive an oblique shock intended to initiate the PBX 9502. Shocks of sufficient strength generated an initiating layer, or region of delayed reaction (relative to typical PBX 9502 detonation reaction timescales) in the PBX 9502 immediately adjacent to the PBX 9501. These reactions result in a transition to detonation away from the 9501/9502 interface in a process analogous to the shock-to-detonation transition in shocked one-dimensional (1D) explosive configurations. The thickness of the PBX 9501 layer was varied from 0.5-2.5 mm to control the strength and duration of the transmitted shock into the 8 mm thick PBX 9502. Phase velocities at the explosive outer surfaces, wave front breakout shapes, and post shock particle velocity histories associated with the detonating and initiating zones in the two explosives are reported and discussed. The initiating layer thickness decreased with increasing PBX 9501 thickness for tests with PBX 9501 thicknesses larger than 1.0 mm. A 1.0 mm thick PBX 9501 slab was not able to initiate detonation in the 8.0 mm thick PBX 9502 slab. Further decreasing the PBX 9501 thickness to 0.5 mm resulted in detonation throughout both slabs, with no initiating layer due to the intersection of each explosive's thickness effect curve at this condition. Initiating layers exhibited particle velocity profiles characteristic of non-detonating shocks. Measured phase velocities are in good agreement with Detonation Shock Dynamics (DSD) predictions for PBX 9501. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.