The amounts of deuterium retained within the JET vessel after campaigns in the years 1985-9 have been re-evaluated and shown to indicate that codeposition of D and wall material is the principal retention mechanism. The amount retained per pulse (~4x 10exp20 D atoms per pulse on average) is shown to fit well with a model wherein the amount of carbon (or other wall material) passing through the plasma during the discharge determines the maximum retention (which can only be realised if first wall temperatures do not exceed 270 C). Such a model can then be used to predict hydrogen isotope retention in other large machines such as ITER. For ITER if a global impurity level in the plasma of 1% is used, the maximum retention (wall temperature <270 C) of tritium would be 500g in 10exp6 secs.