This work describes measurements of the heterogeneous depositional nucleation of ice on micrometer-sized (NH4)2SO4 (AS), maleic acid (C4H4O4) (MA), and mixed AS/MA particles and on thin NH4NO3 (AN) films. The critical saturation ratio (Scrit) required for heterogeneous nucleation was found to be similar and temperature-dependent for all solids, ranging from Scrit = 1.42 ¿ 0.04 at 190 K to Scrit = 1.04 ¿ 0.05 at 240 K. Deliquescence of the solids was not observed in any experiment; ice nucleation always occurred below the deliquescence relative humidity. The observed saturation ratios for ice nucleation were all substantially lower than the saturation ratios required for homogeneous nucleation. These results suggest that vapor deposition of ice may compete with deliquescence for crystalline organic and inorganic solids below the eutectic temperature.