( Co _x Pt _1-x)_1-y Si _y alloys with Si content from 1 to 20 at. % have been grown over a range of growth conditions. Co-deposition of even trace amounts of Si with Co–Pt alloys causes the growth-induced magnetic anisotropy and chemical clustering found in these vapor-deposited alloy films to decrease or vanish. It also causes significant reduction in grain size. Addition of 5 at. % Si eliminates anisotropy completely. Addition of 1 at. % Si produces a film with magnetic properties identical to pure Co–Pt alloys grown at lower deposition temperatures. We suggest this suppression of anisotropy and related effects are due to a decrease in surface mobility during growth in the presence of even trace amounts of Si. © 1999 American Institute of Physics.