The present invention relates to a process for the preparation of a polyolefin, wherein ethylene and/or propylene are polymerised in a liquid or supercritical hydrocarbon reaction medium of a polymerisation reactor R1 in the presence of a solid catalyst composition and an antifouling composition, the solid catalyst composition comprising
(i) a transition metal compound of formula (I)

         L m R n MX q      (I)

wherein
M is a transition metal of anyone of the groups 3 to 10 of the periodic table (IUPAC),
each X is independently a monovalent anionic σ-ligand,
each L is independently an organic ligand which coordinates to the transition metal M,
R is a bridging group linking two organic ligands (L),
m is 1, 2 or 3,
n is 0, 1 or 2,
q is 1, 2 or 3,
m+q is equal to the valency of the transition metal M, and
(ii) a cocatalyst comprising a metal M' of Group 13 of the periodic table (IUPAC),
the solid catalyst composition having a porosity measured according to ASTM 4641 of less than 1.40 ml/g,
and
the antifouling composition comprising an organic compound which has at least 14 carbon atoms and comprises at least one polar functional group.