In situ studies on the growth and evolution of platinum nanocrystals in solution were carried out using synchrotron-based X-ray diffraction (XRD) techniques. Ex situ low- and high-resolution transmission electron microscopy (TEM) were used to investigate the nanocrystal morphologies through the different growth stages. In a reaction with low precursor concentration, both XRD and TEM results show that growth occurs at a relatively slow rate and yields faceted morphologies, which are characteristic of a thermodynamically controlled regime. In contrast, the platinum nanocrystals in the high-concentration reaction form branched structures and grow at much greater rates under a kinetically controlled regime. Additionally the growth mechanism of the high-concentration reaction involves a morphology transformation from octapod-like shapes to porous nanostructures, which is brought about by a novel mechanism involving selective growth and etching processes that occur simultaneously and at comparable rates.