What percentage of Vitamin C is in this product?

You asked the same question I had. The red flag I got was that the product name has 100% AFTER Vitamin C and BEFORE the word Serum.... which likely means that it is 100% serum but not necessarily 100% Vitamin C. I asked my dermatologist and she told me that 10% to 20% is the "highest allowable" amount of concentration that is generally safe and effective for skin care - anything higher could cause irritation. Since this is a 'Vitamin C" product, I think the percentage of Vitamin C should not be considered "proprietary" - though the percentages of the other ingredients would likely be proprietary to protect their formula from being duplicated. Generally, ingredients are listed highest concentrations first, and as you go down the list, the amounts would be less and less. The wording on the label is misleading and I'm sure many people think they are buying 100 % Vitamin C. Since this company will not state what the amount is, I will not be buying it.

does this have Phenoxyethanol?

Not that I can see in the list of ingredients

Is this water or oil based?

It’s oil based. It rubs in to skin nicely with out feeling greasy.

I regret to inform you there is no percentage amount of Retinol listed on my label. However, I switched to another brand several months ago, which does not contain Retinol. It is MB (Meera's Beauty) Vitamin C Serum, which has only 20% Vitamin C, hyaluronic acid amino blend and Vitamin E. I prefer it over the Concentrated Naturals brand.