Problem statement: Evidence of rejection or acceptance by phytophagous pests of citrus plants treated with mineral oils suggests that impa cts of oils may be related to detection by olfactor y and/or contact chemoreceptors of repellent oils or plant volatiles. Alternatively, the oils may mask o r suppress attractant host plant volatiles. To date, no studies have reported that oils elicit an olfact ory response in Diaphorina citri Kuwayama (Hemiptera: Psyllidae). Therefore, we undertook two studies to determine how oil deposits on citrus leaves infl uenced the behaviour of D. citri . Approach: The attractiveness of citrus leaves treated separately with an nC21 horticultural mineral oil and an nC24 agricultural mineral oil to D. citri was evaluated under laboratory conditions. In the first of two studies, the oils were applied as 0.5% aqueous emul sions to mandarin leaves and responses of adults determined after they walked on surfaces saturated with either distilled water or 2% aqueous emulsions of the oils. In the second study, responses of adul t psyllids to host-leaf volatiles, mineral oil vola tiles emanating from leaves dipped in 2% v/v aqueous emulsions of each oil and similarly treated filter paper paired with water-dipped leaves were determined in single, linear-tube olfactometers. Results: In the first study, deposits of both oils significa ntly reduced the proportion of D. citri attracted to citrus leaves. Prior exposure of the insects to oil deposi ts did not influence this response. However, prior contact with the HMO slightly reduced the proportio n of psyllids alighting on the water-dipped leaves. In the second study, oil volatiles emanating from f ilter paper significantly reduced the attractivenes s of water-dipped mandarin leaves. Conclusion: These results indicate that application of the oil s to the mandarin leaves may have: (a) suppressed release of attractant host plant volatiles; (b) masked attractant host plant volatiles; (c) led to the rel ease of repellent volatiles from leaves and/or (d) to adults being repelled by oil volatiles. The results from the study using the olfactometer suggested th at detection of mineral oil volatiles by olfactory rec eptors of adult D. citri contributed to their response to oil deposits on the mandarin leaves.