Selected acyclic disubstituted η⁴-(1E,3E)-dienyl-Fe(CO)₃ iron complexes presented a non-covalent, but reversible affinity for multi-walled carbon nanotube (MWCNT) sidewalls likely mediated by hydrophobic and/or π–π stacking interactions. Resulting iron-complexed MWCNT-based composites displayed methyl ester/acetate, aldehyde and/or OH functional groups on CNT sidewalls. They displayed strong FT-IR absorption peaks due to organometallic CO groups of the Fe(CO)₃ unit (νCO = 1966–2068 cm⁻¹). These peaks appeared in an absorption window free of any parasitic band that usually characterize organic/bio-organic species. Routine FT-IR spectroscopy enabled an effective tracking of adsorption processes of iron complexes onto MWCNT sidewalls. Iron-complexed MWCNTs may be readily dissociated using CH₃CN, allowing an accurate weight quantification of adsorptions. Both interacting components remained unmodified after composite dissociation. Sidewall oxygenated defects at high concentration were also shown to be detrimental to iron complex adsorption. This quantitative decoration methodology may constitute a quite unusual application in the Quality-Control (QC)-driven standardization/improvement of the industrial production of MWCNTs (detection of MWCNT sidewall oxygenated defects during fabrication and purification).