The ash content of coal can be determined by a method based on pair production. Goal is irradiated with high energy γ-rays and the resulting 0.511 MeV annihilation and Compton scattered γ-rays are measured. The pair production (PP) technique has been previously proved in the laboratory on static bulk samples and in the field on high-througput sample by-lines. In the present paper, a plant test to assess the PP gauge for direct on-line conveyor belt analysis is described. This test wasundertaken on the recirculating coal facility at the pilot plant coal washery at the BHP Steel Works, Newcastle, New South Wales. Seven Hunter Valley coals with ash in the range 7.5–33 wt% were circulated around the conveyor loop, and scanned by both PP and low energy γ-ray transmission (LET) gauges. Samples were measured on-belt as a function of sample depth, compaction, moisture and particle size. The mass per unit are of coal on the belt was varied in the range 40–210 kg m −2. The r.m.s. deviation between PP gauge ash and chemical laboratory ash was 1.07 wt% for 370 individual on-belt measurements on coal of mass per unit area greater than 60 kg m −2 and 0.45 wt% ash for the mean ash ofeach sample.