Chemical equilibrium theory analogous with dissociation of molecules is applied to high-T c superconductors. The starting point are preformed pairs, which exist in the normal state and can be treated as Coulomb bosons with charge 2e. Above T c the pairs (B 2 + ) decay into fermions (h + ) according to the equilibrium reaction B 2 + 2h + . Using an approximate chemical equilibrium constant proportional to pressure P, we derive for the pressurized optimum transition temperature a universal two-parameter expression T c M A X (P)/T c M A X (0)=(1+β 1 P) 1 / 2 (1-β 2 P) 2 , where β 1 (>0) and β 2 are parameters. We have applied this formula to the mercury compounds, where we obtain β 1 =0.093(GPa) - 1 . This then shows that in an interesting pressure range P>10GPa, the expansion in powers of pressure diverges, due to the square root. One can also derive a more general doping formula T c (x,P)=A(1+β 1 P) 1 / 2 [x-x 1 (P)][x 2 (P)-x].