The lipophilicity of twenty new diquinothiazines has been determined by reversed-phase thin-layer chromatography on RP-18 silica plates with acetone-aqueous TRIS (tris(hydroxymethyl) aminomethane) buffer as mobile phase. The R M values were linearly dependent on the concentration of acetone. The R M0 values were determined by linear extrapolation to 0% acetone. The experimental lipophilicity log P TLC was determined by use of a calibration plot obtained for six standards. The calculated lipophilicity log P calcd was obtained by use of the software ClogP and compared with experimental log P TLC values. Structure-lipophilicity relationships are discussed on the basis of the log P TLC values and the correlation between R M0 (intercept) and b (slope) of the TLC equation.