This data relates to the development of a modified pre-concentration method for mercury (Hg) stable isotope measurements that uses chemical Hg reduction and gold trap amalgamation followed by semi-rapid thermal desorption (less than 1 hour) and chemical trapping in a novel oxidizing solution. The thermal desorption method was demonstrated to perform adequately on multiple trapping matrices previously presented, however our new bromine monochloride (BrCl) wet oxidant trap (40% 3HNO3:BrCl) was deemed superior. As such, this solution was capable of trapping consistently in 2mL volume over a wide range of Hg masses (5 to 200 ng). The procedure was also shown to work effectively on natural matrices, waters and sediments, producing comparable isotope results to the direct digestion analyses. Here we present the data from all of the experiments conducted during method development for this technique. Table information: T1-Data Dictionary T2- Mercury isotopic results and trapping efficiencies for NIST 3133 standard using different trapping matrices (oxidants) for the 40 minute desorption program. T3- Mercury isotopic results and trapping efficiencies for NIST 3133 standard using the new bromine monochloride and nitric acid oxidant with the 40 minute desorption program. T4- Mercury isotopic results and trapping efficiencies for a certified sediment material over varied purging times which with the aid of chemical reduction strip Hg gas from digestions. T5- Mercury isotopic results and trapping efficiencies for assorted natural matrices using the 40 minute desorption program and the bromine monochloride oxidant trap. Data presented in this data release relates to the journal article: Janssen et al. Rapid Pre-Concentration of Mercury in Solids and Water for Isotopic Analysis, Analytica Chimica Acta https://doi.org/10.1016/j.aca.2018.12.026