Sue - I'm going to assume that you are mixing the powder with water and keeping it no more than a week for each batch.  I've never seen a study on these but I think I wouldn't mix it with sunscreen or moisturizers because it would change the pH of the Vitamin C which would alter drastically how easily it can be absorbed by your skin cells.  Also, the ingredients in the creams could interact with the Vitamin C in ways that are unpredictable.  Why not just mix it and put it directly on the skin, let it absorb a bit and then apply your moiturizer, etc.? Best, Dr. Irwin