The question of whether immobilised proteins are bound covalently on plasma activated or deposited polymers has been a challenge for almost 30 years, as there has been no directly evidence to conclude unambiguously. In this paper, we report evidence that a chemical binding reaction occurred during immobilisation of microperoxidase‐11 (MP‐11) proteins on plasma polymer surfaces. Untreated polymer surfaces were not resistant to detergent cleaning and did not show any increase of S 2p component after protein immobilisation, whilst all plasma‐deposited surfaces showed large amounts of immobilised MP‐11 proteins after detergent cleaning. We conclude that the MP‐11 protein had a chemical binding reaction thorough its cysteine residues with the plasma polymer surfaces.