Laser diffraction particle sizing is increasingly used in grain-size analysis. Commercially available laser analyzers have a number of practical advantages such as detailed particle sizing over a broad size range, built-in ultrasonic dispersing system, and short time of analysis. Grain-size measurements based on laser diffraction analysis (LDA) are characterized by high-repeatability and high accuracy. However, there is no standardized test procedure especially because the research methodology should depend on the type of land. LDA-based measurements of fine-grained sediments rich in clay minerals can contain untrue information due to high anisotropy of participle shape. This causes the optical particle diameter measured by the laser is much larger than that determined as the equivalent spherical diameter in traditional sedimentation techniques for grain-size analysis. This results even in twice reduction of clay fraction quantity, while the silty particles are overestimated in relation to traditional techniques based on Stoke’s law. The aim of this article is to assess the possibilities of using laser diffraction methods for the identification and differentiation of clayey sediments with common origin. The article provides the results of analyses of Neogene clays characterized by significant lithological differences with respect to the participation of clay fraction particles. The research was conducted on clay samples taken in Bydgoszcz. Significant differences in the content of clay fraction particles were found in relation to the applied methods of grain-size analysis. However, as statistical tests showed, results obtained using the LDA method could be useful to characterize lithological variability within the tested soils.