Abstract: The specific surface distribution of a pulp may be obtained by pumping a sample at a pressure to overcome the pressure drop of a plurality of hydrocyclones arranged in series with the apex of leading hydrocyclones connected to the inlet of the next hydrocyclone, etc. in the series. Back pressure is maintained on the base outlet of the leading hydrocyclone, the consistency of the feed to the first hydrocyclone is sufficiently low to ensure that the consistency of the fraction rejected from the apex outlet of the penultimate hydrocyclone in the series is adequate for efficient separation in the final hydrocyclone of the series i.e. to have a feed consistency not exceeding about 0.25% to the last hydrocyclone in the series. Samples are collected from outlets from the hydrocyclones and may be tested to determine specific surface thereby to obtain an indication of the specific surface distribution of the pulp.