In this study, the effect different amounts of Ag and Co have on thermal stability, phase formation and mechanical properties of the glass-forming alloys (Cu0.5Zr0.5)95-x-yAl5AgxCoy (2 ≤ x ≤ 5 and 0 ≤ y ≤ 2) and Cu47.5-yZr47.5Al5Coy (0 ≤ y ≤ 2) was investigated, respectively. The addition of Ag increases the glass-forming ability (GFA) of the alloys by enhancing the stability of the undercooled melt and by simultaneously reducing the stability of the B2 CuZr high-temperature phase. Co instead, has the opposite effect. It leads to an increased thermal stability of the B2 phase, as well as a decreased stability of the undercooled melt and, consequently, lowers the GFA of the alloys. Moreover, a metastable big cube phase precipitates in some alloys stabilized by oxygen, which is introduced during casting. There is a strong interdependence between the phase formation and the tendency of the present alloys to vitrify. This correlation can be captured by the K-parameter (Song et al. 2011 [1]) calculated from the respective transformation temperatures. The plastic strain of the present ternary, quaternary and quinary alloys reflects the GFA as well: the higher the tendency to form a glass, the smaller the plastic strain. Once B2 crystals precipitate in the glass, the plasticity is enhanced significantly.