Abstract: A new vitamin B₁-selective liquid membrane electrode with thiamine-tetra(mmethylphenyl) borate ion-pair complex as active material is proposed. In aqueous solutions at pH 2～4, the electrode showed Nernst response over the Vitamin B₁ concentration range 1×10^-1M～1×10^-6 M. In the citrate buffered media vitamin B₁ was found to be stable enough even at pH 8, and the electrode showed a two-fold sensitivity in determination with cationic slope of 54 mV/concentration decade, fast response, and good reproducibility. Methods for rapid determination of vitamin B₁ in various pharmaceutical preparations are described. Ka₁ value for protonated thiamine in citrate buffer (I=0.5) was determined from the Slope-pH relation (pK_a1=5.04).