Accurate characterization of the reactive surface area of nanocrystalline materials is a vital yet challenging aspect of understanding their properties. This work demonstrates that the ligand-assisted dissolution of heterogenite (CoOOH) nanocrystals by iminodiacetic acid (IDA) can serve as a probe of reactive surface area. Specifically, two geometric isomers of Co(IDA) 2- result, and the relative concentrations of these isomers change with the particle aspect ratio. In experiments in which CoOOH particles were aged under varying pH conditions, the observed size and shape of the particles are strongly influenced by pH; aging under higher pH conditions results in larger particles with a smaller aspect ratio, which is reflected in the ratio of dissolution products. A model for particle growth is proposed, and the corresponding rate law is fitted to the experimental data. This leads to the conclusion that particle growth occurs predominately by the stacking of very thin primary platelets across the (003) surfaces.