Biomass-derived black carbon (bio-char, charcoal) has been shown to increase crop growth by improving nutrient retention and availability. The advantage of bio-char over other types of organic matter additions is its high stability and recalcitrance in soil. While bio-char is known to persist in soil for long periods of time, the dynamics of its mineralization and change in properties are largely unknown, especially in cultivated soil. We studied the changes in properties and stocks of bio-char over a 100-year period using a chronosequence approach on oxisols in Western Kenya. Fields were selected that were under continuous unfertilized maize converted from primary forest at different times in the past (3, 7, 15, 30, 50, 70, 100 years). The residual bio-char that accumulated during landuse conversion using slash-and-burn was studied by spectroscopic and wet-chemical techniques. Oxidation of bio-char surfaces increased rapidly over the first years, whereas oxidation of entire bio-char particles progressed much more slowly. These results will aid in the quantification of residence times of bio-char in cultivated tropical soil.