Inclusion complex formation of 4-hydroxy-3-methoxybenzoic acid (HMBA) and 4-hydroxy-3,5-dimethoxybenzoic acid (HDMBA) with α-CD, β-CD, HP-α-CD and HP-β-CD were studied by absorption, steady state fluorescence, time resolved fluorescence, FT-IR, (1)H NMR and molecular modeling methods. The effect of the CDs with HMBA and HDMBA were studied in pH∼1, pH∼7 and pH∼10 buffer solutions. The study revealed that both hydroxybenzoic acids formed 1:1 complex with the four CDs. The theoretical values suggest that both guests are partially encapsulated into the CDs cavity. The hydroxy group is present in the interior part of the CD cavity and carboxyl group is present in the hydrophilic part of the CD cavity. Molecular modeling studies proved that (i) the negative Gibbs energy and enthalpy changes for the inclusion complexes indicated that the formation of these complexes were spontaneous and exothermic, (ii) hydrogen bonding interactions played a major role in the inclusion process, (iii) the dipole moment values for guests increased when they entered into the CDs cavities which is an indication of the increase of the polarity and the formation of complex and (iv) differences in binding energy and enthalpy change suggest that the β-CD formed more stable complex than α-CD.