The biennial review of atomic weight, r
(E), determinations, and other cognate data has resulted in changes for nickel from 58.69±0.01 to 58.6934±0.0002 and for antimony from 121.75±0.03 to 121.757±0.003 due to new calibrated measurements. Because the measurement of the isotopic composition of mercury has also been improved during the last two years, the Commission was able to reduce the uncertainty of the atomic weight of this element from 200.59±0.03 to 200.59±0.02. Due to the nearly constant isotopic composition of protactinium in nature, where 231
Pa is the predominant isotope, the atomic weight of this element was fixed to 231.03588±0.000 02. The Table of Isotopic Compositions of the Elements 1989 will be published as a companion paper to that on Atomic Weights of the Elements 1989. The Table of Standard Atomic Weights Abridged to Five Significant Figures and current data on isotopic compositions of nonterrestrial material are included to benefit users who are more concerned with the length of time during which a given table has full validity to the precision limit of their interest. The Table of Atomic Weights to Four Significant Figures was prepared and has been published separately.