Future fusion reactors, like ITER, will rely on an active exhaust system to pump tritium (T) in the divertor and then recirculate it to the fuel stream. Estimation of the T inventory requires a detailed T balance, which determines if T is preferentially enriched relative to D in its pathway from the main plasma to the divertor and pump. On the Joint European Torus (JET), the neutral T concentration in the sub-divertor (pumping plenum and region below the divertor strike point plate) is measured with a modified Penning gauge coupled to a high-resolution spectrometer. In addition, T concentration measurements are made in the plasma edge and strike point region with a spectrometer viewing these regions. The sub-divertor and divertor (region above the strike point plate) T concentration measurements show differences during initial T uptake and retention which are characteristic of wall deposition properties. Since wall retention is one of the factors in calculating the eventual T inventory in a reactor, a detailed study of this process has been undertaken.