Temperature-dependent simultaneous measurement of birefringence and optical rotatory power (ORP) for orthorhombic crystals of potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) has been achieved by using the high-accuracy universal polarimeter method at 632.8 nm wavelength. The birefringence and ORP changes along the (110) planes for KTA and KTP crystals were found to have a nearly parabolic form for the temperature range 297–493 K. The thermal variation coefficients were found to be 0.9 (5) × 10−5 K−1 and 5.6 (3) × 10−8 K−2 for KTA and 0.9 (5) × 10−5 K−1 and 5.7 (3) × 10−8 K−2 for KTP. The ORPs at 297 K were found to be 20.0 (20) and 20.5 (15)° mm−1 for KTA and KTP, respectively. The thermal variation coefficients of the ORP were found to be 1.8 (2) × 10−3° mm−1 K−1 and 1.1 (1) × 10–5° mm−1 K−2 for KTA and 1.9 (2) × 10−3° mm−1 K−1 and 1.2 (1) × 10−5° mm−1 K−2 for KTP.