We report on neutron diffraction measurements on the equiatomic liquid alloy Cs-TI over a wide temperature interval, ranging from 730 to 1700 K, and at pressures up to 100 bar. In the crystalline state, this semiconducting alloy contains octahedrally coordinated polyanions Tl-6(6-). The static structure factor S(Q) of molten Cs-Tl at T = 730 K shows a prepeak at Q = 0.72 Angstrom(-1) indicating intermediate-range correlations and thus the existence of large Tl clusters. With increasing temperature the prepeak shifts towards smaller a values and its height decreases. At T greater than or equal to 1300 K the prepeak in S(Q) vanishes and a strong increase of small-angle scattering is observed, suggesting a loss of coherence between Tl clusters and a tendency towards microphase separation. Analysis of the pair distribution functions n(r) indicates that thallium polyanion clusters may be present in liquid Cs-Tl even up to high temperatures. Based on the experimental diffraction patterns reverse Monte Carlo (RMC) simulations have been carried out. They show that the prepeak and the strong increase of small-angle scattering observed in the experimental total Faber-Ziman structure factors occur predominantly in the partial structure factor S-TlTl(Q). The RMC results support a network-like picture for the arrangement of Tl atoms. With increasing temperature the network disintegrates and the tendency of Tl atoms to form clusters becomes a more general tendency towards microphase separation.