The number of S-S bond in pyrosulphuric acid (oleum) is zero. The compounds contain 4 bonds. Four oxygen atoms are linked to sulphur by double bonds and all others by a single bond.
Oleum or sulfuric acid fuming is a solution of various sulfuric trioxide compounds in sulfuric acid or often called disulfuric acid also known as pyrosulfuric acid. Pyrosulfuric acid is the main constituent of fuming sulfuric acid and is a strong acid and. It is also known as disulfuric acid or oleum which is an oxyacid of sulfur. Pyrosulfuric acid is weaker than sulfuric acid. Salts such as sodium and potassium pyrosulfates are obtained by reacting with the bases.