The relative humidity dependence of the size and scattering coefficient of atmospheric aerosol particles was measured at Washington, DC during the period 26–31 July 1979. Particle growth curves (i.e., curves of the ratio r/r0, of particle radius to particle dry radius, versus relative humidity) were calculated using measured values of the particle composition parameter B ≡ ν¯ϕϵMwρ0/ρwM̄s, where ρ0 is the density of the particle in dry state, ρw and Mw the density and molecular weight of water, ϵ the mass fraction of soluble material in the particle. M̄s the molecular weight of the soluble material, ν¯ the mean number of moles of ions per mole of solute, and ϕ¯ the mean value of the practical osmotic coefficient. To determine B, a mobility analyzer was used to transmit dry particles of nearly uniform size to a thermal gradient diffusion cloud chamber where the supersaturation (Sc,) necessary to activate the particles was measured. Then, a relationship between Sc, r0 and B was employed to determine B. The measurements were made for particles in the size range between 0.03 and 0.08 μm radius. The value of B ranged from 0.10 to 0.40 and averaged 0.23 for the period. For B = 0.23 and r0 = 0.05 μm, the computed values of r/r0 are 1.22, 1.37, 1.57 and 1.90 at relative humidities of 80, 90, 95 and 98%, respectively. The light-scattering coefficient as a function of (decreasing) relative humidity was measured directly by controlling the relative humidity of the air flowing to an integrating nephelometer. A smooth variation of scattering coefficient with humidity was observed. On the average, the scattering coefficient changed by a factor of 3.5 between 30 and 95% relative humidity. In general, the measured dependence of the scattering coefficient on relative humidity was consistent with that computed from the particle growth curves.