An apparatus for measuring flowing suspension includes a radiation source that directs wave motion to the suspension, a flow causing particles of the suspension to be sorted by size. A measuring arrangement measures first values of a first parameter of the wave motion interacted with the suspension at a first wavelength band of the wave motion, and measures second values of at least one of the following: a second parameter of the wave motion interacted with the suspension, and the first parameter of the wave motion interacted with the suspension at a second wavelength band of the motion, the measurement of the first and second values being synchronized. The arrangement forms at least one comparison being configured to relate to one of the first and one of the second values, and forms a distribution with at least one of first values as a function of one of the comparisons.