Vitamin C or L-ascorbic acid powder can be mixed with other products for maximum benefit. Powders will mix best with water-based serums and lotions. Mixing ascorbic acid powder with water and water-based formulations will make it the most effective. However, it can be mixed with some oil-based products to be gentler on the skin. Be sure to mix a fresh batch for every application! This ensures the ascorbic acid does not degrade. If you mix with an oil, consider something like vitamin E.