-Binary mixtures prepared in an homogeneous phase and quenched into a two-phase region phaseseparate via a coarsening process whereby domains of the two phases grow in time. With a numerical study of a spin-exchange model we show that this dynamics first take a system with equal density of the two species to a critical percolation state. We prove this claim and we determine the time-dependence of the growing length associated to this process with the scaling analysis of the statistical and morphological properties of the clusters of the two phases.Phase separation is the process whereby a binary mixture of components A and B, initially in a homogeneous phase, demix. This process leads to the coexistence of two phases: one rich in A and the other in B [1][2][3][4][5][6]. The system, initially in an unstable spatially uniform state, progressively coarsens to approach its thermodynamically stable phase-separated state. Such phenomena arise in binary alloys, fluid mixtures, and polymer blends. Recently, the dynamics of phase separation have seen a revival of interest in the context of experimental [7,8] and numerical [9][10][11][12] studies of binary mixtures of Bose gases.The late time dynamics are well understood. In the absence of driving forces, a dynamic scaling regime with statistically self-similar domain morphology sets in. This regime is well-described by an extension of the Lifshitz-Slyozov-Wagner (LSW) theory [13,14], in which the typical domain radius grows as [15] (whereas for scalar non-conserved order parameter dynamics the growing length is also given by a power law but the exponent is z d = 2). Numerical results in favour of this law were published in [15][16][17] for spin-exchange models although the growth-law can be more complex in particle or polymer phase separating systems, see e.g.[18] and references therein. The pre-asymptotic dynamics leading to this regime have not been discussed in detail in the literature. It was noticed in [19] that the low-temperature evolution of a bidimensional 50:50 binary mixture after a quench from infinite temperature shares many points in common with the one generated by Glauber single spin-flip stochastic dynamics satisfying detailed balance [20,21]. On the one hand, an early approach to critical percolation was noticed, although the time needed to reach this state was not studied in detail. On the other hand, a separation of length-scales in the statistics and morphology of finite size cluster areas and domain wall lengths was observed. Linear or planar objects that are smaller than the typical ones,, satisfy dynamic scaling with respect to d (t), while larger objects were found to be very close to the ones of critical percolation.In this Letter we characterise the early stages of the dynamical process. More precisely, we analyse the way in which the system approaches a state with a stable pattern of critical percolating domains. We monitor a number of observables (to be defined in the main part of the text) and we explain how their behaviour constitu...