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Anatase−brookite composite nanocrystals were synthesized successfully by a controlled sol−gel reaction followed by a prolonged hydrothermal aging or by mild calcinations (300 and 450 °C). The physicochemical and photocatalytic properties of the synthesized TiO2 composites were studied along with several commercially available nanocrystalline TiO2 samples showing different features. Rietveld refinements of the powder X-ray diffraction pattern were used to track the brookite content systematically and to generally assess the phase composition of the different samples and their crystallite sizes. SEM, TEM, and HRTEM were used to characterize the particle morphology, size, and surface faceting. BET/BJH analyses combined with mercury porosimetry determinations were employed to characterize the surface area, porosity, and pore size distribution. The surface state of the TiO2 samples was analyzed by XPS by studying, in detail, the region of oxygen 1s to produce the OH/Otot surface ratio. The photocatalytic activity of all of the samples was tested both for degradation of NO x in the gas phase and for the oxidation of 2-chlorophenol in the liquid phase. The different samples showed the same sequence of activity for the two reactions. The highest degradation and mineralization efficiencies were achieved in the case of samples showing smaller crystallite sizes and larger surface areas. The photocatalytic activity of the anatase−brookite composite, submitted to the hydrothermal treatment, was found to be the highest for both reactions, even greater than that of a single-phase anatase sample showing a much-larger surface area. The different contributions to the photocatalytic performance of the TiO2 nanocrystals are critically discussed.