High-Resolution Transmission Electron Microscopy (HRTEM) results show a strong crystal-chemical and defect dependence on the mode of dissolution of synthetic heterogenite (CoOOH) particles. As-synthesized heterogenite particles are micron-size plates (aspect ratio ∼ 1/30) constructed of crystallographically oriented ∼ 3-nm primary particles or are single ∼ 21-nm unattached heterogenite platelets (aspect ratio ∼1/7). Reductive dissolution, using hydroquinone, was examined in order to evaluate morphology evolution as a function of reductant concentration. Two end-member modes of dissolution were observed: 1) non-specific dissolution of macroparticles and 2) preferential dissolution along misoriented boundaries. In the case of non-specific dissolution, average macrocrystal size and morphology are not altered as building block crystals are consumed. The result is web-like particles with similar breadth and shape as undissolved particles. Preferential dissolution involves the formation of channels or holes along boundaries of angular misorientation. Such boundaries involve only a few degrees of tilt, but dissolution occurs almost exclusively at such sites. Energy-Filtered TEM thickness maps show that the thickness of surrounding material is not significantly different from that of undissolved particles. Finally, natural heterogenite from Goodsprings, Nevada, shows morphology and microstructure similar to those of this synthetic heterogenite.