The collision of X+HX on the collinear SSMK surface is studied by classical trajectory calculations for masses X = 1.008(H), 2.014(D), 3.016(T), and 10.000. Total reactivity maps are given as plots of vibrational phase angle versus relative kinetic energy (range 0.3-2.0 eV). A clear relationship between the maps for different mass combinations is shown. The total reaction probability for T+HT and 10+H-10 shows a range of kinetic energy above the threshold where all trajectories are unreactive, followed by reactive ranges at higher energy. Threshold energies and reactivity maps are discussed in terms of skewed coordinate axes. Product vibrational energy distributions show patterns which are closely related to the total reactivities. The results are interpreted in terms of single and multiple collisions. Copyright