Relieving severe pains is an unmet medical need, in which opioids are of prime importance and in the focus of pharmaceutical companies. The objective of this study is to characterize the physicochemical properties, namely basicity, lipophilicity and permeability of thirty opioid ligands, which include eleven newly synthesized compounds. pH-potentiometry and the shake-flask method were used for the characterization of species-specific basicity and lipophilicity. The effective permeability was determined using a brain-specific parallel artificial membrane permeability assay. Structural modifications, namely O-methylation in position 3, isomerization in position 6, saturation of the double bond in position 7, 14-hydroxylation, and the substitution of N-(β-phenylethyl) and N-cyclobutylmethyl side chains all have various effects on these physicochemical properties, and these are explained and compared to computationally predicted values. Computational predictions inadequately capture hydrogen bond formation with the tertiary amino group in case of 14-hydroxylation, just as the effects of hydroxy oxidation at position 6 and N-methyl substitution with N-(β-phenylethyl). The relationship between lipophilicity, permeability and potency is presented by lipophilic efficiency plots that reveal the most promising compounds. This study emphasizes the importance of experimental determination of these essential physicochemical parameters, furthermore, it can contribute to a more thorough understanding of the pharmacokinetic properties.