The displacive C2/m ⟷ P21/m phase transition that occurs with increasing pressure in cummingtonites has been investigated by means of X-ray single-crystal diffraction at different compositions and degrees of Mg/Fe2+ order. Unit-cell parameters and a sub-set of intensity data of some “b” and “a” reflections were collected for four natural and two annealed samples as a function of pressure. The phase transition is close to second order in character, either with a significant contribution from the sixth order term of the Landau potential or with the fourth order coefficient depending on pressure via the bare elastic constants. The transition pressure, Pc, obtained at the different compositions and degrees of order, displays a plateau at the grunerite end. The evolution of the spontaneous strain associated with the phase transition does not depend on the Mg/Fe2+ substitution or ordering, but it is very sensitive to the impurity (Al, Na, Ca) content.