The first complete investigation of the structure of the selective oxidation catalyst TS-1 is presented. Constant wavelength powder neutron diffraction data collected on isotopically substituted titanium silicalite (TS-1) samples, with a Si:Ti molar ratio of 39:1, have been studied using a combination of single and multiple data set Rietveld analysis exploiting the scattering length contrast between the different titanium isotopes and silicon. This has allowed both the silicon vacancy and titanium site substitution distributions to be determined, which has not been possible previously. Both distributions are found to be nonrandom with the titanium preferentially substituting on 3 of the 12 crystallographically independent framework sites - T8, T10 and T3 (in order of decreasing titanium content) - and silicon vacancies on 2 framework sites - T1 and T5. This study illustrates the power of isotopic substitution in powder neutron diffraction experiments to yield enhanced structural information in complex systems.