Boron isotope substitution in boron‐rich lattices is important because many properties of such materials are structure sensitive. There are measured the unit cell expansion at the room temperature and temperature‐dependence of thermal expansion for β‐rhombohedral boron, when 11B atoms are replaced by 10B atoms. Theoretical analysis of experimental results associates the observed changes in structures of samples of various isotope compositions with isotope effects rather than with technologically concomitant impurities.