The influence of two different comonomers incorporated separately at 1 %w/w into a poly(N-isopropylacrylamide) microgel is investigated by photon correlation spectroscopy, transmission electron microscopy and high sensitivity differential scanning calorimetry. The synthesis, characterisation and subsequent binding affinity of iron protoporphyrin IX to the microgels was also investigated. Mossbauer and electronic absorbance spectroscopies were used to characterise the resulting protoporphyrin IX iron(III) polymer complexes. Co-polymer microgels containing either sulfide or imidazole residues acted as a matrix for the binding of haems to form soluble molecular structures containing multiple iron centres. (C) 2003 Elsevier B.V. All rights reserved.