We present results on the spectral variability of two Galactic black hole X-ray binaries, GRO J1655-40 and GX 339-4, in the hard state. We confirm a transition in behaviour of the photon index with luminosity, such that the well-known decrease in X-ray photon index with decreasing luminosity only continues down to Lbol~ 0.01LE. Below this point, the photon index increases again. For Comptonization models, this implies that the ratio of the Compton luminosity to seed photon luminosity, ?h/?s, changes with bolometric luminosity, consistent with a scenario where seed photons change from cyclo-synchrotron at the lowest luminosities to those from a truncated disc. Alternatively, the transition could mark the point below which the non-thermal jet starts to dominate, or where reprocessed photons replace the viscous photons in an outflowing corona model.
