Kau Bay, a 470-m-deep basin, is separated from the Pacific Ocean by a shallow, 40-m-deep sill. During the last glaciation this sill was above sea level, and the bay turned into a freshwater lake. After glaciation, the sea level rose and the bay turned saline again. Since then, chloride from the newly formed saline sediments diffused into the freshwater sediments. As the sedimentation rate was constant throughout the Holocene it is relatively easy to model the chloride concentration in the sediment pore water. The diffusion coefficient of 35Cl is slightly higher than the diffusion coefficient of 37Cl, thus variations in δ37Cl were expected in the pore water. Because the pore-water history of Kau Bay is well known it was possible to use δ37Cl variations to determine the diffusion coefficient ratio (D35/D37) for the two stable isotopes of chlorine. It was found that this ratio is 1.0023 in the pore water from sediments in this bay.