Three hybrid solar/fossil-fuel endothermic processes, in which fossil fuels are used exclusively as the chemical source for H-2 production, and concentrated solar radiation as the energy source of high-temperature process heat, are considered: (1) the thermal decomposition; (2) the steam-reforming; and (3) the steam-gasification. A seeond-law analysis is performed for establishing their maximum exergy efficiency and CO2 mitigation potential vis-a-vis the conventional combustion-based power generation. These hybrid solar thermochemical processes offer viable and efficient routes for fossil fuel decarbonization and CO2 avoidance, and further create a transition path towards solar hydrogen. (c) 2005 Elsevier Ltd. All rights reserved.