We have studied the rate constants and product branching percentages of the reactions of O- with a large number of alkanes : ethane, propane, butane, isobutane (2-methylpropane), pentane, isopentane (2-methylbutane), neopentane (2,2-dimethylpropane), hexane, heptane, octane, isooctane (2,2,4-trimethylpentane), decane, and dodecane. The efficiencies for the reactions of O- with ethane, propane, and butane increase with increasing size, becoming collisional for butane at room temperature. A small temperature dependence is found. For pentane and larger alkanes, the reactions are collisional at all temperatures, within our uncertainty. In contrast to a previous study, there are two significant reaction channels observed : a hydrogen abstraction channel yielding OH- and a reactive detachment channel yielding neutral products and a free electron. In the normal alkane series, the fraction of OH-produced decreases exponentially with,increasing polarizability, i.e., alkane size. Isobutane and isopentane react similarly to their normal alkane counterparts, while isooctane and neopentane produce significantly more OH- than octane and pentane, respectively. Taken together, the branching fraction results indicate that the reactive detachment channel is due to abstraction of two hydrogen atoms from adjacent carbons within the alkane.