The structure and propagation of the one-dimensional (1-D) detonation for the H-2-O-2 system with argon dilution are calculated by solving the Euler and the diffusive NS equations. Classical detonation modes characterized by simplified reaction model are reproduced, including the highly unstable chaotic detonation, mildly unstable detonation with multi- and single-period pulsations, and stable detonation. For the chaotic propagation, substantial unreacted H-2 is formed behind the front. For the multi-period mode, the amount of the pockets of partially burnt gas is reduced significantly. For the single-period mode, the pocket of partial burnt H-2 is almost not observed, with minor fluctuations of its concentration in the downstream. The role of diffusion in the different unstable detonations is identified and the relationship of diffusion with the pulsating propagation mode is addressed. Diffusion has a relatively prominent role for highly unstable detonation due to the existence of the bulk unreacted H-2 pocket. As the detonation shifts from the highly unstable mode to mildly unstable mode, the effect of diffusion on propagating pulsation weakens. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.