A phase transition gel based on N-isopropylacrylamide was coated on the electrode surface, and heme peptide as a peroxidase model compound was covalently entrapped in the gel. Apparent catalytic activity of the gel for H2O2 electroreduction was controlled by swelling (less than or equal to 30 degrees C) and shrinking (greater than or equal to 40 degrees C) the gel; the shrunken gel exhibited higher apparent activity. Formation of the mu-oxo dimer, or oligomers of heme peptide in the swollen gel and the dissociation in the shrunken gel due to low mobility of heme peptide molecules and steric hindrance, may be responsible for the activity changes. As a result of the comparison with a similar gel containing hemin in place of the heme peptide, it was found that hydrophilicity or hydrophobicity of the active site may affect critically the temperature-dependence of the catalytic activity.