An apparatus is provided for determining particle characteristics, in which a flow path is generated containing particles to be analyzed. A light detection system detecting light received from a measurement zone which has been scattered by the particles. A time duration for which a particle remains in the measurement zone is measured to determine an effective aerodynamic particle diameter and a peak detected received light intensity is measured to determine an effective optical particle diameter. A further particle parameter is also obtained relating to the shape and/or density of the particle. This approach enables more information than only a particle size to be obtained using a single-stage optical analysis system. The additional information may be used to characterize the particles more accurately.