The relationships between powder surface composition and powder rehydration properties under variable conditions of storage are investigated in this paper. A rheological approach was used to evaluate the modifications induced by storage on the rehydration properties of native phosphocaseinate powder. Concurrently, the powder surface composition (i.e., lactose, proteins, and lipids) was evaluated by X-ray photoelectron spectroscopy (XPS). A strong correlation was found between the powder wetting time lengthening and the migration of lipids on the powder surface during storage. XPS studies indicated also an over-representation of lipids on the powder surface (6%) in comparison with total lipids (0.4%) even on fresh powder before storage. Detailed investigation of powder lipids revealed the presence of high levels of polar lipids (66% compared with <1% in milk lipids). Their amphiphilic nature and their melting points could explain the extensive enrichment of lipids observed at the powder surface during processing and storage.