This paper reports the finding of the role of substrate surface chemistry on the interaction between the molten droplets (splats) and solid substrates in thermal spray coating. The substrate surfaces were modified by thermal treatments to grow specific types of oxide and hydroxide layers on the surface. It was found that water released from the dehydration of surface hydroxide triggered by the impact of the droplets provoked splat fragmentation and splashing, resulting in the poor splat-substrate bonding. Although this finding is used to address the question of the nature and the influence of surface adsorbates on the splat formation and morphology in thermal spray coating, it can be applied to other technologies and material processes involving the contact between the molten droplets and metal surfaces. (C) 2010 Elsevier By. All rights reserved.