We examine the role of primordial molecules in the Pop III to Pop II transition. We show that the critical metallicity strongly depends on H 2/HD abundances and thermal history of gas. In a shock processed gas where H 2 and HD are highly abundant the dependence of the critical metallicity on gas density is very steep at n ~ 100 cm -3 and T ≤ 300 K, and can become smaller than 10 -6.
