Lactose and/or lactic acid was added to four acidified dairy formulations before spray drying in a small-scale pilot plant spray dryer, to investigate the effect on powder surface composition and wall deposition during spray drying. Addition of lactic acid significantly reduced the glass transition temperature of powders, whereas lactose had the opposite effect. Addition of lactic acid also led to a significant increase in wall deposition. No significant effect of the formulations was observed for moisture content, water activity and particle size. LF-NMR analysis suggested that lactic acid caused shift of water protons from a more mobile fraction to a tightly bound fraction. Confocal Raman microscopy and X-ray photoelectron spectroscopy (XPS) showed that the particle surface in all powders was dominated by proteins. Adding lactose and lactic acid increased the proportion of both components on the particle surface, thus making powders more susceptible to sticking and causing higher wall deposition.