I could not see any indication of source from any of the closeups of the box provided with the Amazon listing. I went to the Traditional Medicinals website (https://shop.traditionalmedicinals.com/Pau-dArco/p/TDM-000682&c=TraditionalMedicinals@Herbal) and there they also do not give any indication of sourcing.As of now, I suspect they use a combination of inner and outer bark BECAUSE they don't say - I believe if it were inner bark only (that's where the medicinal advantage is) they would say so, and loudly. Using the outer bark is cheaper but much less effective, and so my suspicion is that they do this but try to hide the fact. I would love to be proven wrong on this.