This potent, fast-absorbing formula fights the appearance of dullness and discoloration, increasing radiance and brightness with a 15% infusion of THD ascorbate - a potent, ultra-stable form of vitamin C. **How to use:** Apply one to two pumps of serum to cleansed skin; we recommend using both day and night. This potent, fast-absorbing formula fights the appearance of dullness and discoloration, increasing radiance and brightness with a 15% infusion of THD ascorbate - a potent, ultra-stable form of vitamin C. **How to use:** Apply one to two pumps of serum to cleansed skin; we recommend using both day and night. Sunday Riley uses advanced, clinically proven ingredients blended with balancing botanicals for non-irritating, fast-acting formulas that work to transform the skin.