An in situ crystallization technique with X-ray diffraction analysis complemented by ex situ scanning electron microscopy and chromatographic analysis of the alpha/(alpha + beta) solid-state anomeric ratios has been developed to study the crystallization of lactose/protein mixtures in humid air. This technique was used to determine changes in phase composition and morphology during crystallization. Following an induction period during which water is sorbed, crystallization is rapid and the predominant phase observed using the in situ method in spray-dried lactose/sodium-caseinate, albumin and gelatin is a-lactose monohydrate. However, in the case of spray-dried lactose/whey protein isolate (WPI) the predominant phase that appears is the alpha/beta mixed phase with smaller amounts of a-lactose monohydrate. With pure lactose the alpha/beta mixed phase appears as a transient shortly after the onset of crystallization and a-lactose monohydrate and beta-lactose both appear as stable crystalline phases at longer times. Another transient phase with 2 theta = 12.2 degrees, 20.7 degrees and 21.8 degrees was observed in spray-dried lactose/albumin. This phase decomposed as a-lactose monohydrate developed. Three phases seem to persist in the case of spray-dried lactose/gelatin, namely the phase with peaks at 2 theta = 12.2 degrees, 20.7 degrees and 21.8 degrees, alpha-lactose monohydrate and #-lactose for the duration of the in situ experiment. (c) 2006 Elsevier B.V. All rights reserved.