Determining the composition of an unknown is an early step in determining its structure. Compounds that contain small combinations of bromine, chlorine and sulfur (A + 2 elements) in their molecular formulas produce predicable molecular-ion patterns in their mass spectra. These characteristic patterns have been programmed into an Excel spreadsheet, so that a user need only enter the mass of the lowest-mass molecular ion together with its relative intensity and the intensities of the other characteristic peaks 2, 4, 6, etc amu higher in mass. The spreadsheet returns the correct composition from among 21 combinations of Br, Cl, and S typically found in qualitative organic unknowns. The use of the spreadsheet will be exemplified.