A method and an arrangement for determining properties of fibers in a supension of said fibers with the aid of spectroscopic measurements carried out in a wavelength range within the near infrared range (NIR), comprising introducing the fiber suspension into a measuring cell (20) which is provided with a filter (35) and an outlet (47) at one end thereof; concentrating the fiber suspension in the cell while simultaneously measuring the absorbency until a predetermined value is obtained at that wavelength at which the measurement is effected, or for the highest peak of the registered spectrum, at which point supply of the suspension is stopped, the outlet (47) from the cell is closed and a complete spectrum within the wavelength range is registered, whereafter the sought properties of the fibers are determined in a computer unit on the basis of earlier calibrations carried out on samples of known fiber properties. An arrangement for washing a fiber suspension-sample includes a cylinder having a piston, piston rod and a filter which is mounted close to the end of the cylinder opposite the piston rod, so as to divide the cylinder into a large piston-accommodating chamber and a small chamber, wherein delivery pipes leading to the cylinder and outlet pipes extending therefrom are connected to the small chamber and inlet pipes through which sample is delivered to the cylinder and outlet pipes for washed sample are connected to the large chamber.