A measurement window structure is disclosed for an optical process measurement device. The measurement window structure can include a measurement window made of an optical material and having a measurement surface that is arranged to be placed into a process solution, a sealing surface formed to a frame structure of the optical process measurement device and facing the process solution, the measurement window made of an optical material being arranged to press against the sealing surface, and an attaching device or mechanism for pressing the measurement window made of an optical material against the sealing surface and for attaching it to the frame structure. The sealing surface formed to the frame structure can be a rotationally symmetrical surface and the surface pressing against the sealing surface formed to the frame structure of the measurement window made of an optical material can be a rotationally symmetrical surface.