A crystalline adduct of the anti-tubercular drug, moxifloxacin and trans-cinnamic acid (1:1 molar ratio (MCA)) was prepared to prolong the residence time of the drug in the lungs by reducing its solubility and dissolution rate. Whether the adduct is a salt or cocrystal has not been unequivocally determined. Equilibrium solubility and intrinsic dissolution rate measurements for the adduct (MCA) in phosphate buffered saline (PBS, pH 7.4) revealed a significant decrease in the solubility of moxifloxacin (from 17.68 ± 0.85 mg mL to 6.10 ± 0.05 mg mL) and intrinsic dissolution rate (from 0.47 ± 0.04 mg cm min to 0.14 ± 0.03 mg cm min) compared to the supplied moxifloxacin. The aerosolization behaviour of the adduct from an inhaler device, Aerolizer, using a Next Generation Impactor showed a fine particle fraction of 30.4 ± 1.2%. The dissolution behaviour of the fine particle dose of respirable particles collected was assessed in a small volume of stationary mucus fluid using a custom-made dissolution apparatus. The respirable adduct particles showed a lower dissolution (microscopic observation) and permeation compared to the supplied moxifloxacin. The crystalline adduct MCA has a lower solubility and dissolution rate than moxifloxacin and could improve the local residence time and therapeutic action of moxifloxacin in the lungs.