A simple polarimetric interferometer constructed with a sin gle-mode potassium ion-exchanged (PIE) glass waveguide was used as a disposable HF sensor with an excellent selectivity. By examining the interference between the transverse electric (TE0) and transverse magnetic (TM,) modes during the HF etching of PIE waveguides, the average rate of change in the phase difference was demonstrated to depend linearly on the HF concentration. The compressive stress-induced birefringence in the PIE layer was found to be responsible for the HE-sensing feature of the PIE waveguide polarimetric interferometer.