Nanoparticle dissolution is a common process in synthetic procedures where primary particles are replaced by more stable phases, as well as in environmental settings where they both serve as electron sinks or sources for microbes and are responsible for release of nutrients or contaminants. Iron oxides and iron oxyhydroxides, which are widespread in technological applications and environmental settings, are a prime example. To decipher the pathways that underlie dissolution and quantify the kinetic parameters controlling rates, we investigated dissolution of β-ferric oxyhydroxide (β-FeOOH) nanorods via in situ liquid phase (LP)-TEM. Previous ex situ studies that investigated dissolution of iron (III) hydroxides by proton or photoreductive attack found that photoreductive dissolution of is faster than by nonreductive dissolution (e.g. proton-promoted or ligand promoted thermal dissolution). Other studies concluded that the rate-determining step during dissolution in HCl is protonation of the surface together with formation of a chloride-Fe surface complex. Generally, dissolution of β-FeOOH nanorods in strong acids occurs end-to-end along the  direction and has been attributed to proton penetration and destabilization of so-called “tunnel structures” by removal of Cl ions. In our work, we find dissolution of β-FeOOH nanorods occurs both end-to-end (along ) and side-to-side ( along ) directions under the electron beam, even at extremely low electron dose rates. Dissolution involves two reaction steps: (i) beam radiolysis of the water to release H+ that promotes end-to-end dissolution; (ii) beam induced reduction of Fe(III) at the nanorod surface with subsequent release of Fe(II) into the solution, which drives side-to-side dissolution. To further understand the role of protons on dissolution, we investigated the effect of pH. The results show low concentration pH buffers reduce and even stop end-to-end dissolution, but side-to-side dissolution was still observed, leading to formation of dumbbell-shaped nanopartcles that eventually dissolve completely. Nanorods dissolution was totally inhibited for pH buffer concentrations higher than 100 mM. In addition, we found that chloride ions can inhibit end-to-end dissolution of β–FeOOH nanorods while promoting side-to-side dissolution at low concentration pH. The results provide strong evidence that radiolysis contributes to the reduction of Fe(III) to release of Fe(II) even at neutral pH, thus promoting dissolution of iron oxyhydroxides. Because, photolysis is a naturally occurring process in the environment that is similar to radiolysis, the findings of this in situ study may help to understand dissolution and transformation of iron oxides and iron oxyhydroxides in Nature.