Naturally acidic sandplain soils in the lower rainfall (<350 mm annual average) eastern region of the agricultural areas of south-western Australia are deficient in molybdenum (Mo) for grain production of wheat. Liming soils ameliorates Mo deficiency, but it is not an economic option for these soils because they are naturally acidic at soil depths commonly explored by wheat roots. Consequently, Mo fertiliser, usually as Mo trioxide, needs to be applied to wheat on these soils. The residual value of the Mo fertiliser for these soils was not known, so was measured using grain yield of wheat in 2 long-term field experiments. The Mo fertiliser treatments were applied once only in different years to plots not treated with Mo in a previous year. In both experiments, the residual value of the fertiliser was measured in 1993. Thus, it was possible to determine the effectiveness of the fertiliser applied once only 1–11 years previously (previous Mo) relative to freshly applied (current) Mo applied in 1993. At both sites, a continuous decline in the effectiveness of previous Mo relative to current Mo was related to time of Mo–soil contact. In experiment 1, the effectiveness of previous Mo relative to current Mo decreased by about 40, 50, 60 and 70% when applied 2, 5, 7 and 11 years previously. In experiment 2, on a more acidic soil with a larger capacity to sorb Mo, the relative effectiveness of previous Mo decreased by about 60 and 80% for Mo applied 2 and 6 years previously. The concentration of Mo measured in youngest emerged leaf blades was related to 90% of the maximum shoot yield at the time of sampling (diagnostic critical tissue test value) and to 90% of the maximum grain yield (prognostic critical tissue test value). Irrespective of the growth stage of wheat, both critical diagnostic and prognostic values were about 0.07 mg Mo/kg. The concentration of Mo in grain that was related to 90% of the maximum grain yield was 0.02 mg/kg. The reapplication of Mo fertiliser to naturally acidic sands can be made with knowledge of the residual value and use of tissue testing for Mo, particularly when sampled at early growth stages of wheat.