  Primordial chemistry began, at the recombination epoch, when the adiabatic
expansion caused the temperature of the radiation to fall below 4000K. The
chemistry of the early Universe involves the elements hydrogen, its isotope
deuterium, helium with its isotopic forms and lithium. In this contribution I
will discuss the influence of the primordial molecules on the cosmological
decoupling. In the framework of the gravitational instability theory, each
protostructure started as a tiny local overdensity. As long as these
inhomogeneities are small, their evolution can be studied by the classical
linear perturbation theory. Once the deviations become large, the linear theory
is no more valid. we present the role played by these molecules on the
transition between the linear regime and the non-linear regime, and show that
the molecules can lead to a thermal change at the turn-around point between
these two regimes.
