A relatively simple experimental technique is proposed and demonstrated for making measurements of absolute dewpoints and relative deposition rates from flowing combustion gases containing condensible inorganic vapors (here alkali sulfates). In using this “flash evaporation” technique, one first accumulates an inventory of condensate on a Pt-ribbon target maintained below the dewpoint and then flash-evaporates this condensate inventory into the filament wake, where its alkali content is monitored by alkali-atom emission spectroscopy. As implemented here, this method is shown to have a number of important advantages over more time-consuming alternatives (e.g. gravimetric) or demanding alternatives (e.g. optical reflectance interferometry or ellipsometry-). In particular, the flash evaporation technique can also detect liquid condensate inventories which are small enough to be negligibly influenced by surface run-off produced by inevitable gas-side shear stress and liquid condensate surface tension gradients. Illustrative Na2SO4and K2SO4-deposition rate data, and corresponding dew-point data have been obtained in a series of alkali-seeded propane/air atmospheric pressure flat flames, and are shown to be in quantitative accord with expectations of a simple convective-diffusion theory disallowing condensation within the thermal boundary layer, but assuming equilibrium at the vapor/condensate film interface. This class of experimental methods appears to be well-suited to the study of multicomponent condensate vapor deposition, and, with provision for active probe cooling, can be used to explore the effects of condensation (formation of a highly dispersed second phase) within thermal boundary layers, even for unary systems.