Tungsten is one the primary candidate materials for the high neutron flux, high temperature components of a future demonstrate fusion reactor. Despite this, there is a lack of data on W under fusion relevant neutron doses and irradiation temperatures. Transmutation reactions result in the production of Re and Os solute atoms, at a rate which is highly sensitive to the neutron energy spectrum. Under a fusion neutron spectrum the transmutation rate is considerably lower than in typical fission spectrum neutron irradiation experiments. In this study, single crystal and polycrystallineWsamples irradiated at the High Flux Reactor (HFR) at 900 ýC were characterised using Atom Probe Tomography (APT). Bulk chemical and isotopic concentration predictions were validated by analysing the mass spectrum from APT experiments. A post irradiation composition of W-1.263,0.149at.%Re- 0.083,0.017at.%Os-0.009,0.004at.%Ta was measured in the single crystal sample, whereas W- 1.090,0.067at.%Re-0.080,0.016at.%Os-0.009,0.004at.%Ta was measured for the polycrystalline. APT showed that a high number density of Re and Os rich precipitates had formed due to neutron irradiation. These typically presented with a core rich in Re and Os, surrounded by a less dense Re rich cloud. Analysis of the cluster composition was performed using a conventional Maximum Separation Method (MSM) and using an automated method for taking compositional line profiles, orientated with the z-axis, through the centres of precipitates defined using a MSM. MSM gave an average cluster composition of 8.6,3.4at.%Re-0.6,0.6at.%Os and 6.9,3.6at.%Re-0.6,0.5at.%Os for the single crystal and polycrystalline samples respectively. Automated line profile analysis gave a considerable higher average cluster composition of 24.6,12.4at.%Re-3.4,2.9at.%Os and 19.5,6.7at.%Re- 1.8,1.4at.%Os. APT showed that in the centres of some of the precipitates, a dense rod-shaped Re and Os rich region was correlated with these higher compositional measurements in the line profile analysis. Line profile analysis suggests that in the centre of the precipitates, the threshold composition for phase formation may have been reached, as has been observed on a larger scale in higher transmutation rate experiments. Line dislocations and dislocation loops were observed to be decorated with both Re and Os, in agreement with predictions from DFT.