Photocatalysts have focused on scientific endeavors for five decades already. Their ability to generate solar fuel via relatively environmentally benign processes brings promises of a future with increasingly sustainable energy production. A class of materials, heterojunction (HJ) semiconductors, have immense potential due to their versatility, stability, and cost-effectiveness. In addition, meticulous engineering of p-n HJ enables the apparition of an electric field at the junction, a supplementary driving force that drives the charge carriers to separate effectively upon illumination. Therefore, we combined n-type carbon nitride with p-type CuGaO2 to form a photo-active p-n HJ. Mechanistic insights being highly sought-after, we then employed X-ray photoelectron spectroscopy and X-ray absorption spectroscopy as complementary and orbital-specific techniques to probe the changes caused by interfacing CuGaO2 with g-C3N4.