Abstract: A new phenytoin-selective liquid membrane electrode with phenytoin cetyltrioctylammonium (in n-heptanol) as active material is proposed for rapid determination of phenytoin, The linearity slope decreases with decreasing molecular weight of the active material studied in the order: phenytoin cetyltrioctylammonium>phenytoin dodecyltriheptylammonium>phenytoin cetyltrimethylammonium>phenytoin tetrabutylammonium. For homologs of alcohols and esters of phthalic acid, the linearity slope decreases with elongation of the carbon chain.