The phase transformation behavior of TiO2 sol-gel synthesized nanopowder heated in a sealed quartz capillary from room temperature to 800 degrees C was studied using in-situ synchrotron radiation diffraction (SRD). Sealing of the capillary resulted in an increase in capillary gas pressure with temperature. The pressures inside the sealed capillary were calculated using Gay-Lussac's Law, and they reached 0.36 MPa at 800 degrees C. The as-synthesized material was entirely amorphous at room temperature, with crystalline anatase first appearing by 200 degrees C (24 wt% absolute), then increasing rapidly in concentration to 89 wt% by 300 degrees C and then increasing more slowly to 97 wt% by 800 degrees C, with there being no indication of the anatase-to-rutile transformation up to 800 degrees C. The best estimate of activation energy for the amorphous-to-anatase transformation from the SRD data was 10(2) kJ/mol, which is much lower than that observed when heating the material under atmospheric pressure in a laboratory XRD experiment, 38(5) kJ/mol. For the experiment under atmospheric pressure, the anatase crystallization temperature was delayed by similar to 200 degrees C, first appearing after heating the sample to 400 degrees C, after which crystalline rutile was first observed after heating to 600 degrees C. The estimated activation energy for the anatase-to-rutile transformation was 120(18) kJ/mol, which agrees with estimates for titania nanofibers heated under atmospheric pressure. Thus, heating the nanopowders material under pressure promoted the amorphous-to-anatase transformation, but retarded the anatase-to-rutile transformation. This behavior is believed to occur in an oxygen-rich environment and interstitial titanium is also expected to form when the material is heated under high gas pressure. This suggests that atmospheric oxygen appears to accelerate the amorphous-to-anatase transformation, whereas interstitial titanium inhibits TiO2 structure relaxation, which is required for the anatase-to-rutile transformation.