We show that there exist two phases in the chemical equilibrium state of dark interstellar clouds, according to the local value of the density, n H , and cosmic ray ionization rate, ζ. For a given value of ζ, the ion-neutral chemistry at high density is driven by proton transfer with H 3 + while at low density charge transfer with H + dominates. The transition between these two regimes occurs abruptly. Much higher values of the ratio n(C)/n(CO) are attained just below the critical density than just above it. The value of the critical density increases with ζ