An SEM-EDS instrument sensitive to C and O is used to measure cation ratios in individual fluid inclusion decrepitates of main paragenetic phases from two Nova Scotian Mississippi Valley-type deposits. The technique confirms that the analyses obtained for the carbonate hosts are not contaminated by substrate and permits measurements of small amounts of decrepitates. The technique has been used to investigate fluid compositions in inclusions of presulfide-stage dolomite and calcite, sulfide-stage sphalerite and calcites, and postsulfide calcite at the Gays River and Jubilee Zn-Pb deposits in Nova Scotia. The results indicate that Na and Ca are the two major cations. Minor amounts of K and Mg are also detected. NaCl and CaCl 2 ranges for decrepitates from the two deposits are as broad as for modem mixed brines and thereby suggest that solution mixing partly controlled the paleobrine compositions. When compared to other Mississippi Valley-type deposits, the Gays River and Jubilee deposits are similar in NaCl-CaCl 2 -KCl composition to a group of Mississippi Valley-type districts including deposits from the Ozark region that are associated with large volume clastic sequences or paleoaquifers. Higher proportions of KCl at Gays River and higher proportions of CaCl 2 at Jubilee indicate that the mineralizing brines interacted with Horton Group clastic aquifers of different mineral compositions. The brines in the two areas might have originated from evaporation of seawater, but they were later modified by interaction with clastic rocks of different compositions.