Real-time ultra-small-angle X-ray scattering (USAXS) was used to follow the flocculation of titania particles in two paint systems as the paint films were drying. The inter-particle scattering was extracted by comparing the time series with diluted titania dispersion (having negligible inter-particle interaction). The paint system with pigment affinic groups showed considerably less flocculation than a pure acrylic emulsion. The results were confirmed by scanning electron microscopy (SEM) images of the dried film. The likely cause of this difference and utility of USAXS for such measurements are discussed. (C) 2009 Elsevier Inc. All rights reserved.