Thirteen isothermal ternary diffusion experiments have been performed for a single composition of the system raffinose-KCl-H2O at 25 degrees C. Both Rayleigh and Gouy optical systems were used in all experiments for direct comparison of results from these two configurations. Various data analysis procedures are compared. Good agreement is found between the recommended results from the two optical methods, although diffusion coefficients D-ij obtained by the Rayleigh method are generally slightly lower than those obtained by the Gouy method. The sensitivity of the D-ij to both experimental error and choices of subsets of the 13 experiments is discussed. Standard errors of coefficients are an inadequate measure of accuracy. This study has led to higher standards for acceptability of individual experiments. This system has the same mean solute concentrations as the four experiments reported by Dunlop, for which a test of the Onsager reciprocal relations (ORR) failed. Our diffusion results are reasonably similar to those of Dunlop. Therefore the reported failure of the ORR test probably results from errors of estimating activity coefficients for this system father than from diffusion data.