I HAVE obtained a new determination of the packing fraction of 79Br by measurements of the doublet given by its triply charged atom and the quintuply charged atom of 132Xe, the value for which has been provisionally given1. As the difference in mass between 79Br and 81Br has also been measured2, values for both isotopes can be calculated as given below. These are provisional only, but are certainly more reliable than those previously in use. I hope in the future to get more accurate results by means of the doublet. 81Br+++ – C2H3, but this cannot be used now owing to the presence of aluminium in the discharge tube.