The mechanism of liquid-liquid phase separation and factors determining the solid-state microstructure of monotectic alloys are discussed. The effect of the cooling rate on the phase-separated morphology is shown in examples of Al-In, Al-Pb, Ni-Nb-Y and Zr-Gd-Co-Al alloys solidified by different techniques. A remarkable improvement of the microstructure for the Al91Pb9 hypermonotectic alloy cast with TiB2 particles, which catalyze the phase separation, is demonstrated.