Abstract The surface composition of spray-dried lactose-containing powders was estimated by electron spectroscopy for chemical analysis (ESCA), and particle structure was studied by scanning electron microscopy before and after storage in a humid atmosphere. The surface composition of dry powders made from lactose/sodium caseinate solutions was nearly identical to that of powders stored at 75% relative humidity for 4 d. Protein started to dominate the powder surface even when it was present at only low solution concentrations. The particle structure after storage in a humid atmosphere was completely changed even though the surface composition remained relatively unchanged. When soybean oil emulsions with sodium caseinate and lactose were spray-dried, protein was over-represented on the powder surface. If the protein concentration was sufficiently high, the fat was almost completely encapsulated. In the case of the powders with high lactose content almost total release of encapsulated fat on the powder surface was observed after storage in a humid atmosphere. When the lactose concentration was lower, release of fat on the powder surface was less pronounced. The structure of fat-containing powders was studied by scanning electron microscopy. Before storage in a humid atmosphere, particles were discrete with smooth surfaces. As the fat was released onto the powder surface, the powder became highly agglomerated and attained a smooth structure.