A laboratory method was developed, which facilitates to investigate how the temperature profile during a TMP-process including a bleaching stage influences pulp brightness. Thin shavings are prepared by slicing a fresh and frozen block of wood with a microtome. The shavings are rapidly dried in an air-stream at room temperature and the optical properties are measured in an Elrepho instrument. After that the shavings are placed in a stainless steel container that can be pressurized with steam, where the temperature profile in a TMP-process or a bleach plant can be simulated. Addition of bleaching chemicals can be done during the simulation. The optical properties are measured and can be compared to those of the same shaving before treatments. The temperature profile in a TMP-process has a large impact on the optical properties of the pulp. Many studies have been done to show how brightness is affected, when temperature/pressure in TMP processes are varied. However, pilot-scale or full scale-trials can often be performed only under certain limitations. With this new method it is easy to simulate different processing conditions and measure the kinetics of the brightness change on a shaving sample before and after treatments. To get as high final brightness as possible it is necessary to limit the brightness reduction as much as possible in manufacturing of the unbleached pulp.