Hollow gold nanospheres (HGNs) with nearinfrared (NIR) surface plasmon resonance (SPR) absorption are highly desired for many applications including photothermal ablation therapy (PTA) of cancer; however, they are challenging to synthesize at relevant resonant wavelengths in a reproducible manner. In this work, we have systematically varied the reaction parameters to determine the origin of the irreproducibility of synthesis. This allows for much finer control of the synthesis, including homogeneous NIR absorbing HGNs that were characterized using UV−vis spectroscopy and electron microscopy (EM) techniques. We have found that cobalt seed particle growth time plays a more critical role than previously realized and is one of the most important parameters for high synthetic reproducibility. The results also provide new insight into the mechanism of cobalt seed and HGN growth, which further aids the successful synthesis of high quality HGNs with strong and tunable NIR SPR absorption.