[en] Homogeneous mixtures of boron and silicon with compositions ranging from 0 to 14.3 at.% Si were reacted at 1600⁰C in an argon atmosphere. X-ray powder patterns of these samples revealed only two regions in this composition range. From 0 to 5.6 at.% Si a regular variation of the cell parameters of β-rhombohedral boron is observed that corresponds to a single-phase solid solution of silicon in boron. In the range 5.6-14.3 at.% Si a two-phase alloy occurs, consisting of a mixture of SiB₆ and silicon-saturated boron solid solution. Micrographic examinations and microprobe analysis of metallographic sections of bulk zone-melted B-Si alloys support these conclusions. Density measurements performed on bulk samples with silicon contents varying from 0 to 5.6 at.% indicate that, unlike many other impurities, silicon could dissolve by substitution in β-rhombohedral boron. (Auth.)