Compared to thiols (R-S-H), persulfides are uncommon.  They are thermodynamically unstable with respect to loss of elemental sulfur:
:RSSH → RSH + 1/8 S
Nonetheless, persulfides are often kinetically stable.
The S-H bond is both more acidic and more fragile than in thiols. This can be seen in the bond dissociation energy of a typical persulfide, which is 22 kcal/mol weaker than a typical thiol, and the lower pK of about 6.2 for persulfides compared to 7.5 for thiols. Thus, persulfides exist predominantly in the ionized form at neutral pH. This effect is attributed to the stability of the RSS· radical.