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Fluorescence properties of 5,10,15,20-tetra( m-hydroxyphenyl)chlorin ( m-THPC), a sensitizer used in photodynamic therapy (PDT), were studied in the presence of three native cyclodextrins (CDs) -α, -β, and γ-CD—and two modified cyclodextrins—heptakis (2,6-di- O-methyl) β-CD (Me-β-CD) and heptakis (2- O-hydroxy-propyl) β-CD (HP-β-CD). The CDs studied have been shown to undergo the formation of an inclusion complex with m-THPC, leading to a large enhancement (up to 300 times) of m-THPC fluorescence intensity depending on the CD used. Stoichiometry and association constants have been determined on the basis of the variation of fluorescence intensity. A 1:2 stoichiometry has been found for m-THPC complexes with γ-CD, Me-β-CD, and HP-β-CD. Association constants as high as K = 9.5 × 108 M−2 in the case of m-THPC/2Me-β-CD and K = 2.7 × 109 M−2 in the case of m-THPC/ 2HP-β-CD complex were determined, whereas a lower value ( K = 2.4 × 104 M−2) was found in the case of m-THPC/2γ-CD. Because of the highest fluorescence enhancement (a factor of about 300) observed with Me-β-CD, this compound would be the most suitable to improve the detection of m-THPC in further pharmacokinetics and biodistribution studies.