The isothermal diffusion Onsager coefficients L-ij have been calculated and the Onsager reciprocal relation (ORR) has been tested for 23 compositions of the NaCl + Na2SO4 + H2O system at 298.15 K. The calculations are based on our previously reported extensive set of Fick's law based diffusion coefficients measured using the high-quality Gosting diffusiometer and volumetric measurements made with a vibrating-tube densimeter or (at a few compositions) with pycnometry. These calculations also require the four chemical-potential concentration derivatives, which were obtained by reanalysis of available isopiestic data using a hybrid thermodynamic model. This model uses an extended form of Pitzer's ion-interaction model to represent the single-salt thermodynamic activities but uses the mixing terms from Scatchard's neutral-electrolyte model. Because of the ORR, the two cross-term diffusion Onsager coefficients should be equal, i.e., L-12 = L-21, on either the solvent-fixed or volume-fixed reference frames. The ORR was actually tested using the experimental volume-fixed diffusion coefficients, with an equation that reduces the propagation-of-errors calculations by removing unnecessary terms. The ORR is found to be satisfied for 22 of the 23 compositions when estimated errors (usually 5 to 10 % for the chemical-potential derivatives, depending on the composition, 4 times the standard errors for diffusion coefficients or scatter observed from cross-plots, whichever is larger) are assigned for the input quantities.