Abstract: According to the response properties of piezoelectric crystal quartz sensors to the solution conductivity the frequency shift response was derived to show a linear dependence on the concentrations of vitamin B₁ (VB₁) and vitamin C (VC). This was experimentally verified and a calibration model for simultaneous determinations of the two vitamins was established. The usefulness of the technique was evaluated by quantitation of mixtures of unknown composition using common multiple linear regression (MLR) and partial least squares (PLS). The average relative standard deviations for six samples were 4.46% for VB₁ and 6.63% for VC with MLR; and 1.97% for VB₁ and 2.74% for VC with PLS, respectively.