Perovskite-type RRh3B and RRh3C (R = Y, Sc) form a continuous solid solution, RRh3BxC1-x, in the range of 0 ≦ x ≦ 1 with cubic structure (space group: Pm3m, Z = 1). The values of the microhardness of YRh3BxC1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are investigated as 4.4 ± 0.1, 4.9 ± 0.1, 5.5 ± 0.2, 6.4 ± 0.2 and 7.5 ± 0.15 GPa, respectively. On the other hand, the values of the microhardness of ScRh 3BxC1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 4.5 ± 0.2, 6.1 ± 0.2, 7.4 ± 0.2, 8.9 ± 0.2 and 9.6 ± 0.1 GPa, respectively. Thus, the microhardness of RRh 3BxC1-x continuously becomes larger with increasing boron content. The oxidation onset temperatures of YRh 3BxC1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 604, 631, 655, 687 and 978 K, respectively. On the other hand, the oxidation onset temperatures of ScRh3BxC1-x for x = 0, 0.25, 0.50, 0.75 and 1.00 are 674, 675, 695, 725 and 753 K, respectively. Thermogravimetric analysis of the phase indicates that the oxidation onset temperature also increases with boron content. Thus, it appears that both mechanical strength and chemical stability of the RRh 3BxC1-x phase essentially depend on its boron content. Ab initio calculations have been performed to obtain the equilibrium lattice constants and the bulk moduli. The calculated lattice constants are in excellent agreement with experimental results. {\textcopyright} 2005 Elsevier B.V. All rights reserved.