The diffusion charging of aerosol particles by unipolar ions has been studied experimentally over the pressure range, 0.0311 to 0.960 atm, using monodispersed aerosols of di‐octyl phthalate. The ratio,a/&lgr;, of the particle radius to the mean free path of ions has been varied over the range from 1.0 to 66. Within the accuracy of the experiments, the particle charge has been found to be independent of the ratio,a/&lgr;, and well represented by the equation,np=9.00Dpln (1+1.02×10−5N0tDp), wherenpis the number of elementary units of charge acquired by the particle,Dpis the diameter of the particle in microns,N0is the number concentration of the ions to which the particle has been exposed in ions/cc, andtis the charging time in seconds. This equation agrees with a theoretical equation derived by White when the molecular weights of the positive ions used in the charging experiments are assumed to be equal to 16 times that of air or 460.