The efficient design, operation and scaleup of novel liquid fluidized-bed bioreactors for direct coal liquefaction require detailed knowledge of reactor axial pressure drop, dispersion of the solid and liquid phases, and particle size distribution as a function of axial position. In this paper, a fluorescence imaging method is introduced and validated which enables direct in situ visualization and sizing of individual coal particles in a liquid fluidized bed. This is achieved by causing the continuous phase to fluoresce while leaving the particles optically opaque. The technique is validated through its agreement with invasive light-scattering techniques and its ability to report constant size distributions as a function of axial position in a nonsegregating liquid fluidized bed of bituminous coal 45-63 mum in diameter. The method’s ability to detect particle segregation in a bimodal, segregating bed (bituminous coal 45-63 and 106-150 mum) is also demonstrated. Unlike previous sizing and visualization methods, the proposed technique is noninvasive and does not require tracer particles, specialized sample distributions or specialized reactor geometry. Further, it may be utilized for any solids volume fraction and does not require physical or chemical alteration of the solid phase. The segregation information obtained using such a method will enable the accurate development of predictive mathematical models of bioreactor operation for coal solubilization/liquefaction.