A novel simple spectropolarimetric interferometer was developed based on single-mode potassium ion-exchanged (PIE) glass waveguides that generally have a large birefringence due to the compressive stress induced in the ion-exchanged layers. By using the spectropolarimetric interferometry, wavelength dependence of the modal birefringence of single-mode PIE waveguides was accurately obtained in a broad bandwidth, without need to measure individual modal indices. The modal birefringence decreases with increasing wavelength. The spectropolarimetric interferometer was demonstrated to be responsive to changes occurring within the penetration depth of the evanescent field. A refractive-index change of Deltan = 0.002 was easily detected in the case of a 2-cm-long interaction path length.