Single-phase icosahedral AlMnSi alloys containing up to 30 at.% Mn and 33 at.% Si have been prepared directly by quenching from the melt. Alloys of the series Al70-xMnxSi30 (10≤x≤30), Al80-xMn20Six (6≤x≤40) and Al67-xMnxSi33 (28≤x≤32) were investigated. The regions of x for the formation of single-phase icosahedral alloys by direct quenching are x ≈ 26, x ≤ 20 and x ≈ 29 for the three series respectively. These alloys have quasi-lattice constants in the range 0.437-0.461 nm. Details of the structural properties of these alloys as well as their thermal stability as a function of composition are discussed. © 1990.