A simple two-fiber probe has been modified and improved for in situ spectroscopic measurements that utilize frontsurface geometry. The efficiency of the probe for fluorescence measurement across a window is improved by maximizing the overlap of the light cones of the two fibers at the outside surface of the window where the sample is in contact with the window. This is achieved by polishing the ends of the fibers at predetermined angles and using different probe geometries. Improvement factors up to 14 have been obtained for fluorescence of phenol in distilled water, anthracene in 75% methanol, and the solutions mixed with sands and clay. The improved twofiber probe can be used for fluorescence as well as phosphorescence, Raman, and infrared (reflectance) spectroscopic measurements.