The reversed-phase TLC retention behavior of 26 structurally diverse basic and neutral drugs has been investigated under different chromatographic conditions by varying the aqueous component and pH of the mobile phase. Phosphate buffer, phosphate-buffered saline, morpholinepropanesulfonic acid at pH 7.4, phosphate buffer at pH 11.0, and pure water were used with different proportions of methanol as the mobile phase. Use of n -octanol as mobile phase additive was also investigated. Different sets of extrapolated R Mw values were compared and the effect of electrolytes on retention is discussed. Retention was correlated with lipophilicity log P and log D 7.4 . Use of log P with an ionization correction term Q at pH 7.4 led to good correlation and confirmed a reduced effect of protonation on retention. Finally, linear solvation energy relationships were established for all data sets. Apart from the reduced effect of ionization, hydrogen-bond basicity and volume were found to effect to an almost equal extent ...