Covalent and noncovalent attachment of an iron porphyrin (FeP) on the surface of single-walled carbon nanotubes (CNTs) are addressed by density-functional-theory calculations and molecular-dynamic simulations. We investigate the stability and electronic properties of several CNT-FeP assemblies in order to shed light in the experimentally reported electrocatalytic activity of carbon-supported Fe macrocycles and the role of the linking structure. Two mechanisms for the FeP attachment on metallic and semiconducting CNTs were considered: by physisorption through π-π stacking interaction and by chemisorption through s p2 and s p3 bonding configurations. Our results suggest that FeP covalently linked to metallic CNTs would be the best electrocatalytic systems due to its metallic character at room temperature, suggesting that they may work as an electrode with the ability to transport charge to the macrocycle. Semiconducting CNTs would be unlikely because the FeP-CNT assembly preserves the semiconducting character. Noncovalent attachment of FeP onto both CNTs is also unlikely due to the absence of physical contact and the unsuccessful FeP fixation. © 2010 The American Physical Society.