High-resolution transmission electron microscopy (HRTEM) results show a strong crystal-chemical dependence on the mode of dissolution of synthetic heterogenite (CoOOH) particles via ligand-assisted dissolution using iminodiacetic acid (IDA) and reductive dissolution using hydroquinone (H2O) Dissolution, using H(2)Q (10 muM to 2 mM) and IDA (10 muM to 2 mM), of synthesized heterogenite particles (37 mg/L) was examined in order to evaluate morphology evolution as a function of dissolution agent concentration. No evidence for redox reactions was observed in experiments using IDA, and no aqueous complexes of Co(II) or Co(III) with H(2)Q or benzoquione, the oxidation product of H(2)Q, in experiments using H(2)Q were detected. As-synthesized heterogenite particles are micron-size hexagonal plates (aspect ratio, similar to1/30) constructed of crystallographically oriented similar to5 nm primary particles, or they are single similar to 21 nm or similar to 10 nm unattached heterogenite platelets (aspect ratio, similar to1/7 and similar to1/3, respectively). In experiments using the micron-sized hexagonal plates, two dominant modes of dissolution were observed: nonspecific dissolution that dissolved primary building blocks at all locations equally and pathway specific dissolution that occurred along boundaries of misorientation between primary building blocks. Both mechanisms occurred independent of the dissolution agent used. In comparison, TEM results show that dissolution of the unattached heterogenite particles occurs primarily at the {101} and {10 (2) over bar}, or "edge," crystal faces and that no significant dissolution occurs at the (001), or "basal," crystal faces. This suggests that the reactive surface area is dominated by edge faces and further suggests that basal,faces are essentially nonreactive under these conditions. Finally, dissolution by IDA produced two dissolved isomers, u-fac Co[IDA](2)(-) and s-fac Co[IDA](2)(-). Experiments using identical solution conditions show that dissolution of the micron-sized plates favors the production of the u-fac isomer while dissolution of the 21 nm particles favors the production of the s-fac isomer.