Equilibrium measurements for the interaction between various substituted phthalocyanine iron(II) and phthalocyanine (III) and carbon monoxide gas in DMSO are reported. The preparation of a novel octaphenyl-di-tert-butylphthalocyanine iron(II) is described. Variation of the phthalocyanine ring substituents, and the oxidation state of the iron metal, altered the sensitivity of the phthalocyanine towards the carbon monoxide by changing the properties of the ring, especially the tendency towards aggregation and the availability of electron density for binding purposes. Gradual spectroscopic changes were observed for the formation of the complex between carbon monoxide and the phthalocyanine derivatives. The equilibrium constant for octa-phenyl-di-tert-butylphthalocyanine iron(II) is 15170 L/mol and for phthalocyanine iron(III) is 2118 L/mol when compared to the reference phthalocyanine iron(II) value of 7447 L/mol. The results indicate that the sensitivity to carbon monoxide increases with an increase in the electron density on the phthalocyanine ring.