Nine alkaloids were chromatographed by reversed phase (RP) TLC on RP-18 with aqueous acetone or aqueous dioxane mobile phases. A variety of mobile phase additives (ammonia, diethylamine (DEA), tetrabutylammonium chloride (TBA-Cl)) were used to suppress ionization of the alkaloids and/or reduce ionic interactions with surface silanol groups. Ion-pair (IP) RP-TLC was also performed with aqueous acetone mobile phases and reagents such as pentane sulphonic acid (PSA), octane sulphonic acid (OSA), or di-(2-ethylhexyl)orthophosphoric acid (HDEHP) as mobile phase additives. In these systems retention of the alkaloids was determined using mobile phases with different modifier concentrations. Relationships between R M and modifier concentration were sought. A linear semilogarithmic equation was fitted to experimental data and used to obtain lipophilicity values R MW ( R M for pure water), the slope ( S ), and φ 0 , the intercept with the x -axis. The retention of standards with known lipophilicity log P was then determined using the chromatographic systems described above and R MW values were calculated. From these experimental data equations relating log P and R MW were created for each system separately and these equations were used to estimate log P exp values for the alkaloids. log P exp , slope ( S ), and φ 0 values obtained by use of different TLC systems were also correlated.