Emergency services were called after 350 litres of diesel oil was spilled from a lorry.
West Sussex Fire and Rescue Service, Highways England and Sussex Police were dispatched to the incident at about 7.40am.
Officers have cordoned both ways from Houghton at the Whiteways Lodge roundabout on the A29 at Bury Hill.
Diversions are in place, traffic is slow in the surrounding areas.
A fire service spokesman said: "A fire engine from Littlehampton was sent to deal with the oil spill.
"We are working with Sussex Police and Highways England to clear the spillage."
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