We have used the ionisation equilibrium equation to derive the electron density in interstellar clouds in the direction to 13 stars .
A linear relation was found , that allows the determination of the electron density from the Mg I and Mg II column densities in diffuse clouds .
The comparison of normalised equivalent width of 12 DIBs with the electron density shows that the DIBs equivalent width do not change with electron density varying in the range n _ { e } = 0.01 \div 2.5 cm ^ { -3 } .
Therefore the DIBs carriers ( 1 ) can be observed only in one ionisation stage , or ( 2 ) the DIBs are arising in cloud regions ( eg .
cores or cloud coronas ) for which we can not determine the electron density .