Abstract: A non-crosslinked biocompatible polymer is formed from a radical polymerisable ethylenically unsaturated zwitterionic monomer containing a sulpho-betaine zwitterionic group and a radical polymerisable ethylenically unsaturated comonomer containing a hydrophobic group selected from C6-24-alkyl, C1-24-fluoroalkyl and siloxane groups. Suitable copolymers are of N,N-dimethyl ammonium-N-propylsulphonate-N-ethyl methacrylate and dodecylmethacrylate. The polymer may be used to coat substrates to render them biocompatible, especially hemocompatible. The hydrophobic groups render the polymer particularly suitable for coating hydrophobic substrates.