A study on the thermal effects on TiO 2 rutile and anatase nano-powders was undertaken and displayed some unusual photoactivity and crystal structure properties. Rutile nano-particles with different crystallite sizes were characterised and the possible effect on activity were investigated. One of the rutile samples appeared to have trace amounts of anatase and was annealed at high temperatures at 1172 K and 1272 K to highlight the thermodynamic stability phenomenon of titania. Parallel to this study, anatase nano-particles were investigated before and after being annealed up to 1022 K. For all the samples used in this work, characterisation was undertaken using micro-Raman microscopy/XRD and Scanning Electron Microscopy (SEM) while photoactivity assessment was made by measuring and monitoring the photodegradation of a mixture of dye methyl-orange (MeO) and nano-powders under UV-light for 3h30 min in suspension. The study revealed that rutile nano-powder samples were thermodynamically stable even at very high temperatures and poorly active but with an unusual photoactive feature. Concerning the anatase samples; SEM investigation revealed a questioning size growth as the samples showed a different particle size depending on the temperature of thermal treatment. It revealed that annealing at 672 K seemed to be a key temperature as the particles change from a polyhedral structure to a two-dimensional structure showing a platelet like shape. The photocatalytic studies of the anatase nano-particles showed a very high activity especially before annealing. This highlighted the fact that the anatase phase can subsist at high temperatures such as 1022 K and exhibit a persistence in photoactivity even though it has decreased significantly after 672 K. SEM analysis was in accordance with the photoactivity investigation. Nevertheless, the most interesting feature of the results emanates from the reaction order study and rate constant analysis taken from the kinetic shape of the graph of the degradation of MeO as a function of the irradiation time for the different particle sized rutile nanoparticles. Here a zero-order reaction was determined and as a consequence raised questions about the theory of the mechanism of the activities of titania in terms of surface chemistry, surface area dependence and photoactivity. For example, for the nano-rutiles the sample with a 25 nm crystallite size was the most active and the sample with the smallest crystallite size (15 nm) was the least active and yet was found to contain trace levels of nano-anatase. This effect was also substantiated by UV absorption and weathering studies on doped isocyanate-acrylic paint films. UV analysis clearly shows that the absorptivity of the nanoparticles plays a role and correlates with the photoactivity. The 15 nm particles have decreased absorptivity in the near UV and hence decreased activity.