Hygroscopic aerosols were studied at 40° C at different relative humidity levels in a flow-type reactor chamber. The main interest was in growth of hygroscopic aerosols under higher humidity conditions. The time development of the aerosol in the 0.01- to 17-µm size range was determined using electrical aerosol analyzer and optical particle counter aerosol analyses. Low velocity and laminar flow were used to facilitate the comparison with theoretical considerations. Cesium hydroxide (CsOH) and sodium hydroxide (NaOH) were used as hygroscopic materials. Cesium is one of the most abundant species in core melt release, and NaOH is well known for its hygroscopic properties. The primary particles were produced by a constant output atomizer. The dry particle size, as volume median diameter (VMD), for CsOH was 1.8 µm. The observed airborne particle size after 2 min of travel was 6.3 µm, after 5 min 5.3 µm, and after 10 min 3.4 µm at saturated conditions. For dry NaOH aerosol, the measured initial VMD was 2.7 µm. At saturated conditions, the observed VMD was 6.7 µm after a 10-min travel. Theoretical calculations with the modified NAUA code showed that during travel through the chamber, the particle size change can be attributed to hygroscopic growth and sedimentation.