The E 0 +- X 0 + transitions of BiH and BiD have been recorded in absorption. The result of this experiment, together with older data, has been used in model-fitting calculations to show that the low-lying states X 0 +( 1Σ+) and A 1( 1Π) could be described in the case (c) analogies of a 3Σ - state. It has been found that Raynes' equations for a 3Σ state with centrifugal distortion corrections to the spin-orbit coupling constant, λ, best describe the observed term values of the ground state. Two different methods for determining molecular constants (the term value approach and the direct approach) have been used. Comparison shows that there are no obvious differences between the results of the two methods in the present case.