Photodecomposition of methyl– and ethyl–CoIII complexes of meso‐tetrakis(4‐sulfonatophenyl)porphyrin (CH3– and C2H5–CoIIITPPSs) was used as a reaction probe to study the cage effect of cyclodextrin capsules formed by two per‐O‐methylated β‐cyclodextrin (TMe‐β‐CD) molecules and their covalently linked dimer, Ph2CD. The photodecomposition of CH3–CoIIITPPS under aerobic conditions was markedly suppressed in the presence of TMe‐β‐CD and Ph2CD, while C2H5–CoIIITPPS was less affected. Alkyl–CoIIITPPS formed two types of inclusion complex with Ph2CD, the alkyl groups in Type 1 being located at the opposite side of the phenyl linker of Ph2CD and those in Type 2 being located at the same side. The photodecomposition of C2H5–CoIIITPPS in Type 1 proceeded via an ethylperoxo complex, while that in Type 2 occurred via a radical pair generated in a narrow, rigid cage to form ethylene and CoIITPPS.