The heterogeneous ice nucleation efficiency of a series of thin C3–C6 monocarboxylic acid films between 180 and 200 K has been investigated using a Knudsen cell flow reactor. At each temperature, the critical ice saturation ratio for depositional nucleation as well as the effective contact angle was found to be strongly dependent on the chemical nature of the film. For the organic acids used in this study, increasing the O:C ratio lowered the supersaturation required for the onset of heterogeneous ice nucleation and decreased the effective angle of contact. This could be the result of an increase in surface hydrophilicity, which allows the film to better adsorb a metastable, icelike layer of water that serves as a template for the new phase of ice. These ice nucleation results are in excellent agreement with ice nucleation on laboratory generated α-pinene secondary organic aerosol as well as on predominantly organic particles collected in Mexico City.