Relative transition probabilities of spontaneous emission in the R and P branches of the A¹Π → X¹Σ⁺ (0-0) band of BH molecule have been obtained for the first time. It was observed that measured ratios of the line strengths are in agreement with corresponding ratios of Hönl-London factors. Thus, the non-adiabatic effect of perturbations is negligibly small and Hönl-London formulas may be used for derivation of rovibronic population densities from measured line intensities of the (0-0)R branch of BH. The (0-0)P branch is less prospective because too much overlapping by other BH emissions cannot be avoided with the moderate resolution spectrometers usually used in plasma spectroscopy. General considerations are illustrated on the example of a low-pressure plasma of a planar microwave discharge in a H₂-Ar-B₂H₆ (64:33:3) gas mixture (total pressure 1-2.5 mbar, power 1.2-2.4 kW). The values of the rotational temperatures obtained directly for the excited electronic-vibrational states d³Π_u ^-, ν′ = 2 (T_d2 ^rot(H₂)) and A¹Π⁺, ν′ = 0 (T_A0 ^rot(BH)) and those calculated in the framework of excitation-deactivation models for the ground X¹Σ_u ⁺, ν = 0 vibronic states (T_X0 ^rot(H₂) and T_X0 ^rot(BH)) have been analysed. It has been observed that T_X0 ^rot(H₂) and T_X0 ^rot(BH) are in excellent agreement, while compared to these values T_A0 ^rot(BH) is noticeably higher and T_d2 ^rot(H₂) is about two times lower. Thus, the simultaneous measurements of intensity distributions in the rotational structure of BH and H₂ emission bands can be used as a prospective method for determination of gas temperature in non-equilibrium boron and hydrogen containing plasmas.