An olefin polymerisation process carried out in the presence of an anti-fouling agent and a chromium-type catalyst or Ziegler Natta catalyst; characterised in that the anti-fouling agent comprises an anti-fouling polymer having an average molecular weight (Mw) of greater than 1000 daltons and containing: (1) one or more blocks --(CH.sub.2--CH.sub.2--O).sub.k-- where each k is in the range from 1 to 50; and (2) one or more blocks --(CH.sub.2--CH(R)--(O).sub.n.sup.- where R comprises an alkyl group having from 1 to 6 carbon atoms and each n is in the range from 1 to 50, and terminated by a R' and a R'' end groups, wherein R' is OH or an alkoxy having from 1 to 6 carbon atoms and R'' is H or an alkyl having from 1 to 6 carbon atoms.