The complete optical gyration tensor of monoclinic pure L-arginine phosphate monohydrate (LAP) and deuterated LAP (d-LAP) crystals has been determined. Variation of the optical activity in the presence of birefringence of d-LAP crystal with pure LAP is found from the high-accuracy universal polarimeter (HAUP) method at a wavelength of 632.8 nm. The gyration tensor components in terms of rotatory power were found to be p(11) = -6.6(3) degrees/mm, p(22) = 19(2) degrees/mm; p(33) = 49.7(7) degrees/mm; and p(13) = -43.7(8) degrees/mm for the LAP, and p(11) = -1.2(1) degrees/mm, p(22) = 5.0(4) degrees/mm; p(33) = 41.4(3) degrees/mm; and p(13) = -38.0(4) degrees/mm for d-LAP at room temperature.