A high pressure optical flow cell system suitable for use in a real time optical particle monitoring system. The system is modular, with at least two housings joined together with removable mechanical attachment devices. Inlet and outlet passageways introduce and remove high pressure fluid into a flow cavity located between adjacent housing faces. An o-ring or other compliant member seal is provided between the faces to prevent leaks of the high pressure fluid. At least one optical window is provided with a substantially planar face flush with the flow cavity surface. An optical assembly maintains the face of the optical window flush with the flow cavity surface over a wide range of temperatures and pressures. A system and method for maintaining the face of the optical window flush with the flow cavity surface over a wide range of temperatures and pressures.