We report here on the development of a sol-gel based fiber optic sensor to monitor local blood pH. These sensors are being developed for invasive medical applications; that is, they will be coaxially threaded through a catheter beyond an occlusion in the vascular system. The fiber optic pH sensor design was based on the immobilization of a pH sensitive dye, seminaphthorhodamine-1 carboxylate, onto the tip or surface of an optical fiber using the sol-gel method. The fiber optic pH sensor was tested in phosphate buffer saline and human whole blood using a miniature fluorimeter system. Linear responses were obtained in blood in the pH range 6.7 to 8.0, which demonstrates potential for use for in vivo sensing.