A general formulation is developed for calculating penumbra solar-pressure-gradient torque. A literal expression for the dependence of light intensity on position is given and the three-dimensional gradient about any point of interest in the spacecraft is computed. Solar-gradient torques are studied, via numerical simulation, for both Earth-pointing and sun-pointing planar spacecraft in geostationary orbit. Both specularly reflecting and totally absorbing surfaces are considered. Eclipse conditions are identified for the following critical cases: 1) maximum solar-gradient pitch torque; 2) maximum solar-gradient roll torque; and 3) longest duration within penumbra. The maximum instantaneous torque and angular impulse from solar-gradient torque are compared with those arising from gravity-gradient torque and are shown to be significant for some spacecraft orientations. A comparison of equivalent center-of-mass-center-of-pressure offsets for solar-gradient and conventional solar torque indicates that solar-gradient torque may potentially become dominant for very large spacecraft. It is also argued that the symmetry present within an eclipse season permits an attitude control approach based on angular momentum storage.