This part of ISO 13317 describes and principles and proper use of gravitational photosedimentation techniques for the characterisation of dispersed phases of suspensions and emulsions. These techniques monitor the gravity-induced phase separation of particulate materials dispersed in liquids by recording photometric signals (i.e., intensity of transmitted or scattered light) as function of vertical position and/or measurement time. Recorded signals reflect the changes in local particle concentration due to sedimentation and can be easily transformed to distributions of sedimentation velocity. This computation does not demand essential preconditions and theoretical assumptions. The conversion of velocity into particle size (Stokes diameter) relies on the applicability of Stokes’ law. As fractionating technique, sedimentation analysis facilitates to distinguish between particle fractions of close sedimentation velocity. Accordingly, particle size distributions can be very finely resolved, which is an advantage compared to spectroscopic ensemble techniques. The measured distribution functions are intrinsically weighted by photometric quantities (e.g. light extinction or scattered light intensity). Yet, they are frequently converted into volume or number-weighted distribution functions by employing established models for light-particle interaction. Gravitational photosedimentation facilitates the granulometric characterisation of dispersed materials of non-zero density contrast to the continuous phase, incl. solid particles and emulsion droplets. The available measurement range depends on dispersed and continuous phase properties and typically amounts to 200 nm – 100 μm for aqueous samples, whereas sedimentation velocity can be quantified for the range 0,6 µm/s – 10 mm/s. Also, the working range with regard to particle concentration is strongly affected by material properties and by particle size, yet it is typically well below 1 vol%. The data analysis relies on the assumption that all particles have the same density and comparable shapes and do not undergo chemical or physical change in the continuous phase. In addition, gravity photosedimentation by scanning, or time and position resolved extinction allows to characterize the dispersion state regarding, e.g. clarification, segregation, agglomeration, consolidation and physico-chemical stability. Photosedimentaion is equally applicable to determine particle density as well as sediment and cream layer formation. NOTE. This part of ISO 13317 can involve hazardous materials, operations and equipment. This part of ISO 13317 does not purport to address all the safety problems associated with its use. Explosion proof analysers are required when examining volatile liquids with a low flash point. It is the responsibility of the user of this part of ISO 13317 to establish appropriate safety and health practices and to determine the applicability of the regulatory limitations prior to its use.