We present here for the first time the one pot reversible and quantitative interconversion of three crystalline polymorphs of a compound by ball mill grinding at milling equilibrium, Scheme 1. The fact that all three polymorphs are crystalline proves unambiguously that the product of ball milling is uniquely and reproducibly defined by the milling conditions. Our model disulfide compound, 2nitrophenyl-4-chlorophenyl-disulfide, here called 1-2, has been investigated by our team for various purposes together with a few related phenyl-disulfide compounds with different functional groups on the phenyl ring. The number "1" refers to the 2-nitrophenylpart, and (2-nitrophenyl)2-disulfide is thus called 1-1; while the number "2" corresponds to the 4-chlorophenylpart, and (4chlorophenyl)2-disulfide is thus called 2-2. Form B is the bulk thermodynamic polymorph of 1-2 under ambient temperature and pressure, and can be easily recrystallized from a range of solvents (see ESI Section 3.5 for details). Recrystallized crystals of Form B were used for the polymorph turnover experiments presented in this paper. Form A is obtained by milling under NG conditions from Form B or the newly-discovered Form C while Form B can be obtained from Form A under LAG with a range of solvents, Scheme 1. and from Form C by LAG with MeCN. Form C can be obtained from Form B or Form A by LAG with water. Details of the crystal structure solution of Form C, introduced for the first time in this paper, are reported in the ESI Section 2.2. We can quantitatively interconvert these three polymorphs (Form A, Form B and previously unknown Form C) by modifying the experimental ball mill grinding conditions using NG or LAG. Figure ) to prove that these polymorph transformations can be repeated as many times as desired and are reproducible.