AbstractDarkening of the fines and fiber fraction of peroxide-bleached thermo-mechanical pulp (TMP) was studied on a laboratory scale using a device that simulates the short circulation of a paper machine. Selected chemicals were dissolved in the circulation water to study the effect of different cations and anions. Ferrous ions darkened the fines and fiber fraction by increasing the light absorption. It is very important to keep the iron content of the pulp and white water as low as possible because even low contents of iron have a negative effect on the brightness of produced paper. Also, chloride ions decreased the brightness by increasing the light absorption of the fines and fiber fraction. Calcium ions increased both the light absorption and light scattering of the fines fraction. The increased light absorption dominated and the brightness decreased. Calcium chloride is used very commonly as a model chemical for calcium ions in laboratory tests, but the use of it is not recommended, if the darkening phenomena are studied, because chloride ions decrease the brightness. Aluminum ions increased the brightness of the fines fraction by increasing the light scattering. Manganese, sodium, sulfate, or carbonate ions did not affect the optical properties.