Interdiffusion coefficients of the systems KNO2 + H2O and KClO3 + H2O at 25 degrees C were measured by free-diffusion optical-interferometric methods from low concentrations to near saturation. Density and viscosity data were also obtained for these systems. Low concentration conductance data were obtained for the KNO2 + H2O system at 25 degrees C for calculation of the,limiting molar conductance of the NO2- ion. The diffusion coefficients of KClO3 + H2O dropped over the whole concentration range as concentration increased. This was similar to the concentration dependence in the same concentration range for KNO3 + H2O and KHCO3 + H2O at 25 degrees C. The diffusion coefficients of the system KNO2 + H2O initially dropped, plateaued, and dropped again as concentration increased. The diffusion results were compared to data of other potassium salts.