Experimental data on the thermodynamics of liquid, amorphous and crystalline phases of Zr-Cu and Zr-Ni systems are analysed from the point of view of amorphization ability of these systems and thermodynamic stability of forming amorphous alloys. The scope of the associate solution model is shown for description of the thermodynamic functions of formation of liquid Zr-Cu and Zr-Ni alloys in wide temperature ranges, as well as for description of the thermodynamic functions of amorphous phase. The ranges of glass formation in Zr-Cu and Zr-Ni systems correlate with the composition of the ideal associate solution, which was calculated for the temperatures of the liquidus line in the corresponding phase diagram. Thermodynamic stability of amorphous alloys and glass forming ability of undercooled liquids are evaluated in terms of their Gibbs free energies.