Ice formation by black carbon particles was studied using a continuous flow thermal diffusion chamber. Submicron particles were treated to uptake H2SO4 in amounts estimated to range from zero to several percent by weight. The particles were processed at constant temperatures (-40 to -60 ¿C), and humidity from ice saturation to values exceeding water saturation. Conditions required for ice nucleation were determined for different acid exposures. Untreated soot particles showed activity as deposition/sorption ice nuclei. Particles with approximate monolayer equivalent coverage by H2SO4 nucleated ice at humidities for which it was inferred that the acid was dilute enough to freeze homogeneously on soot surfaces. Particles with multi-layer equivalent H2SO4 coverage froze as solution droplets most readily, Heterogeneous freezing of the latter particles was observed to occur in preference to the homogeneous freezing of an externally mixed population of H2SO4 droplets at lower temperatures. ¿ 1999 American Geophysical Union