Summary Polyethoxylated nonionic tenzides containing nonyl-phenyl and tributyl-phenyl groups as hydrophobic parts decreased considerably the net potassium influx in roots of wheat seedlings. Their effect depended quadratically on the number of ethyleneoxide groups per molecule and on the molecular lipophilicity determined by reversed-phase thinlayer chromatography. The optimum range of biological effectivity was 6–13 ethyleneoxide groups per molecule. Besides lipophilicity and ethyleneoxide number steric parameter (width of hydrophobic part) also influenced significantly potassium influx. These variables explained about 78 % of total variance. Polyethoxylated sorbitane derivatives had no effect on the potassium influx.