Abstract
The solidus boundary in the 
pseudobinary system was determined by a technique which employs the electron microprobe to analyze the primary phase of compositions equilibrated in the two‐phase field. The solid solutions in this system appear to be essentially ideal over the entire composition range, since the experimental solidus boundary is in good agreement with that calculated with the assumption of ideal solid and liquid solutions. This is consistent with the hypothesis that an alloy whose two components have the same lattice parameter will obey Raoult's law of mixing.