The first systematic and quantitative experimental study of the influences on lateral drying in colloidal films is reported. The time until water recedes from the edge of a drying thick film of a waterborne colloidal dispersion, called the open. time, was measured as a function of several controllable parameters. Magnetic resonance microscopy, using a specially designed probe, noninvasively provides a direct and quantitative measurement of the concentration of water as a function of vertical and lateral position. Images were obtained from drying films of latices with known values of thickness, particle size, and surface tension, which are neatly encapsulated in an expression for the reduced capillary pressure, p,. A strong increase in the open time was found over a relatively narrow range of p(c) values. Larger particles, slower evaporation rates, and thinner films encourage more uniform lateral drying with a delay in drying from the edges. This observation is consistent with a recent model (Routh, A, F.; Russel, W. B. AIChE J.1998, 44, 2088) based on the lubrication approximation. The experiments and the modeling point to a way of achieving control over the lateral drying processes of waterborne colloids.