A study was made of the feasibility of using single-source electron gun evaporation for the preparation of amorphous Gd-Co alloy films. The sources used covered the compositional range from Gd0.20Co0.80 to Gd0.60Co0.40. The films deposited from each source were analyzed and the results compared with those expected for ideal alloys based on calculations using Raoult's law and Langmuir's equation. It was found that real alloys gave films with compositional gradients more than twice as severe as would be expected from ideal alloy calculations. In order to obtain films with compositional variations of one percent or less, the evaporation must consume no more than one percent of the source material.