The present invention provides, for the first time, a process that meets the needs of the drug industry for measuring the particle size of nebulized aerosols simultaneously or one after another by the laser diffraction method and the cascade impactor method which is known in the art. In this way it is possible to bring the reliability of the results of the laser diffraction process according to the invention into conformity with that of the cascade impactor, and thereby obtain a process which combines the advantages of the rapid laser diffraction process with the accuracy of the otherwise time-consuming cascade impactor method. In addition to the process, apparatus for carrying out the process are also disclosed.