Changes in the composition of International Association for the Physical Sciences of the Oceans Standard Seawater (SSW) for salinity measurements were examined for borosilicate glass bottle-type SSW batches P138 (2000) to P165 (2021) to understand their impact on salinity and density stability of SSW. Results showed that (i) silicate increased with time (5.47 μmol kg−1 year−1); (ii) dissolved inorganic carbon (DIC) decreased with time (−13.5 μmol kg−1 year−1); (iii) total alkalinity, nitrate, nitrite, and phosphate were constant; and (iv) ammonium, dissolved organic carbon (DOC), and total dissolved nitrogen were constant for batches after P148, although they were highly variable for batches before P147. The rate of increase of silicate was similar to that for low-salinity standard seawater (10L-Series) and for the old batches sealed in soda-glass ampoules. A glucose addition experiment was used to examine the effect of DOC on density. The results enable estimation of the Absolute Salinity anomaly (δSA) of SSW as a function of time (t in year) from calibration of SSW with changes of silicate and DIC: δSA (g kg−1) = 0.0024 + 0.00021 t (for t > 0.3 year). For batch P149 (2007), mass fractions of major constituents (Na+, Mg2+, Ca2+, K+, Sr2+, Cl−, SO42−, and Br−) were measured in 2020, and the mass fractions were generally consistent with results from previous studies in 2010 and the mass fractions of the Reference Composition, which is currently the best estimate of the chemical composition of SSW, although it is necessary to improve the accuracy of the measurements to detect temporal variations of mass fractions of major constituents of SSW.