In medicine knowledge of pH values can provide us with information not only about the patients’ status but also about physiological processes in the patient’s body. Measurements of pH in small-sample volumes and online pH monitoring in vivo can be employed to obtain such information. For such measurements we have developed and investigated U-shaped fiber-optic probes with immobilized pH indicators in this paper. U-shaped probes with a diameter of about 2 mm were prepared. Three different pH indicators, methyl red, methyl orange, and bromothymol blue, were immobilized in two types of matrices, namely, porous silica (PS) and ethylcellulose (EC), and applied on the U-shaped probes. Changes in spectra of transmitted power were measured and calibration curves were determined from these spectra. It has been found that a working pH range of prepared probes was from 3.1 to 7.6. The maximum sensitivity was about 0.1 1/pH unit. Effects of structural relaxations of detection layers and indicator leaching observed in experiments are discussed.