A method and system for determining particle size distribution and/or filterable solids in bitumen-containing fluid is described. A sample of bitumen-containing fluid, such as bitumen-froth feed, bitumen-froth solvent or paraffinic-froth-treated (PFT) bitumen-solvent is obtained. An optimized diluent combination is determined, comprising an aromatic or cycloaliphatic solvent such as toluene, benzene, naphthalene, xylene, anthracene, or cyclohexane together with a C3 to C12 paraffinic solvent. The combination is considered optimized when diluting the sample with the combination maintains substantially the same level of deasphalting in the diluted sample as in the undiluted sample. Upon dilution of the sample with optimized diluent combination, particle size distribution can be accurately determined using optical instrumentation, laser diffraction instrumentation, electrical counting instrumentation, or ultrasonic instrumentation.