In the last several decades, significant efforts have been directed toward better understanding the gaseous and Particulate Matter (PM) emissions from aircraft gas turbine engines. However, limited information is available on the hygroscopic properties of aircraft engine soot particles, in particular their soluble mass fraction (SMF). This parameter plays an important role in the water absorption, airborne lifetime, obscuring effect, and detrimental health effects of these particles. This study reports the description, detailed lab-based performance evaluation of a robust Hygroscopic-Tandem Differential Mobility Analyzer (H-TDMA) and subsequent field deployment to measure the SMF of aircraft engine soot particles in the exhaust from CFM56-2C1 engines burning several fuels during the Alternative Aviation Fuel EXperiment (AAFEX) II campaign. The fuels used were a conventional JP-8, tallow-based hydro-processed esters and fatty acids (HEFA), Fischer-Tropsch, a blend of HEFA and JP-8, and Fischer-Tropsch doped with Tetrahydrothiophene (an organosulfur compound). In all cases the SMF was observed to increase with fuel sulfur content and engine power condition. SMF decreseased with increasing particle size. The highest SMFs (~ 80 %) were found in the smallest particles, typically those with diameters of 10 nm.