First, may I congratulate you on your diligence? Using a serum is an excellent idea. It can deliver antioxidants or it can exfoliate, which not only brightens your complexion but also improves the absorption of your moisturizer. So apply a serum to your clean, dry face, wait three to five minutes till it's absorbed, and then put on your moisturizer, says Susan C. Taylor, MD, a dermatologist based in Philadelphia and New York.
Bottom line: Applying a serum doesn't block a moisturizer's absorption.