Recent kinetic studies, particularly those by means of shadowgraphy and X-ray radiography, for supercritical water oxidation of solid carbonaceous and resinous substances have revealed the importance of the O-2 mass transfer process over the intrinsic surface reaction at higher temperatures. The mass transfer processes, internal and external one, should be incorporated in designing SCWO processes for solid substances and related processes such as catalytic SCWO. Some model calculation efforts of late are briefly described. Finally, fundamental information required for future development is itemed. (C) 2008 Elsevier B.V. All rights reserved.