TiO2 samples with different crystal sizes and compositions were synthesized using a sol gel method at different calcination temperatures (350-900 degrees C). The activity and stability of TiO2 samples were determined by the gasification of formaldehyde in supercritical water (SCW) and by treatment in SCW. Increasing calcination temperature and SCW gasification (SCWG)/SCW treatment decreased the surface area of anatase TiO2 samples due to growing crystallite size via agglomeration and sintering. Among anatase TiO2 samples, the TiO2 calcinated at 450 degrees C was found as the most suitable material under SCW conditions. However, the surface area of rutile TiO2 slightly increased from 17.2 m(2) g(1) to 19.8 m(2) g(1) with the weakly crumbling of particles during SCWG. The highest hydrogen formation (63%) from formaldehyde in the SCW was obtained in the presence of anatase TiO2 calcined at 350 ?C and rutile TiO2 calcined at 800 degrees C. CO2 formation in the presence of anatase TiO2 is higher than rutile TiO2 because of the presence of active lattice oxygen species (O, O-2) based on O-2-TPD. (c) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.