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The optical rotatory power of SrS2O6.4H2O was measured at room temperature along and perpendicular to the optic axis for a wavelength λ = 633 nm. The values ρ|| = 2.65 and ρ⊥ = −0.6° mm−1 were obtained, where || and ⊥ mean along and perpendicular to the optic axis, respectively. For the measurements of the optical activity along a birefringent crystal section, the so-called HAUP device (High-Accuracy Universal Polarimeter) has been used. The values of the different optical parameters have been successfully explained using the classical point-dipole–dipole polarizability theory and a recently determined ordered superstructure assigned to the space group P31. In this way, a connection between the absolute structure of the crystal and the sign of the optical rotation has been established. The conclusion is reached that the main contributors to the optical activity are the water oxygen atoms; in particular, the gyration along the optic axis is satisfactorily described by the exclusive effect of these atoms.