To date, fiber diffraction on A-form NaDNA has always shown a B-form contamination. Here we have used optic microscopy, calorimetry, and neutron scattering techniques to define a method to obtain DNA fibres samples whose molecules are purely in the A-form. When the impure sample is heated to 320 K, the DNA molecules in the B-form undergo a transition into the A-form. Our studies have modified the accepted phase diagram for NaDNA films by including the dependence of temperature crucial for the purification of A-form samples by removal of B-form contamination.