A polyolefin composition, said composition comprising: a terpolymer comprising a cross-linkable polyolefin comprising hydrolysable silane groups, wherein the cross-linkable polyolefin comprises a polyethylene containing hydrolysable silane groups, and a polar comonomer, wherein the polar comonomer is selected from (meth)acrylates of alcohols having 1-4 carbon atoms, wherein the polar comonomer is present in an amount of 5-35 wt% based on the total amount of the terpolymer, wherein said polyolefin composition further comprises: a cross-linking catalyst in an amount of 0.0001 to 6 wt% of the total composition, and a silicon containing compound in an amount of 0.001 to 3 wt% of the total composition, wherein the silicon-containing compound has the general formula (R 1) x [Si y (R 3) z] m wherein R 1, which may be the same or different if more than one such group is present, is a monofunctional, or, if m = 2, is a bifunctional, hydrocarbyl residue comprising from 1 to 100 carbon atoms; R 2, which may be the same or different if more than one such group is present, is a hydrocarbyloxy residue comprising from 1 to 100 carbon atoms; R 3 is -R 4 SiR 1 p R 2 q, wherein p is 0 to 3, preferably 0 to 2, q is 0 to 3, preferably 1 to 3, with the proviso that p + q is 3, and R 4 is -(CH 2) r Y s (CH 2) t - where r and t independently are 1 to 3, s is 0 or 1 and Y is a difunctional heteroatomic group selected from -O-, -S-, -SO-, -SO 2 -, -NH-, -NR 1 - or -PR 1 -, where R 1 and R 2 are as previously defined; and x is 0 to 3, y is 1 to 4, z is 0 or 1, with the proviso that x+y+z=4; and m = 1 or 2. a terpolymer comprising a cross-linkable polyolefin comprising hydrolysable silane groups, wherein the cross-linkable polyolefin comprises a polyethylene containing hydrolysable silane groups, and a polar comonomer, wherein the polar comonomer is selected from (meth)acrylates of alcohols having 1-4 carbon atoms, a cross-linkable polyolefin comprising hydrolysable silane groups, wherein the cross-linkable polyolefin comprises a polyethylene containing hydrolysable silane groups, and a polar comonomer, wherein the polar comonomer is selected from (meth)acrylates of alcohols having 1-4 carbon atoms, wherein the polar comonomer is present in an amount of 5-35 wt% based on the total amount of the terpolymer, wherein said polyolefin composition further comprises: a cross-linking catalyst in an amount of 0.0001 to 6 wt% of the total composition, and a silicon containing compound in an amount of 0.001 to 3 wt% of the total composition, a cross-linking catalyst in an amount of 0.0001 to 6 wt% of the total composition, and a silicon containing compound in an amount of 0.001 to 3 wt% of the total composition, wherein the silicon-containing compound has the general formula (R 1) x [Si y (R 3) z] m wherein R 1, which may be the same or different if more than one such group is present, is a monofunctional, or, if m = 2, is a bifunctional, hydrocarbyl residue comprising from 1 to 100 carbon atoms; R 2, which may be the same or different if more than one such group is present, is a hydrocarbyloxy residue comprising from 1 to 100 carbon atoms; R 3 is -R 4 SiR 1 p R 2 q, wherein p is 0 to 3, preferably 0 to 2, q is 0 to 3, preferably 1 to 3, with the proviso that p + q is 3, and R 4 is -(CH 2) r Y s (CH 2) t - where r and t independently are 1 to 3, s is 0 or 1 and Y is a difunctional heteroatomic group selected from -O-, -S-, -SO-, -SO 2 -, -NH-, -NR 1 - or -PR 1 -, where R 1 and R 2 are as previously defined; and x is 0 to 3, y is 1 to 4, z is 0 or 1, with the proviso that x+y+z=4; and m = 1 or 2.