The temperature dependence of the pH for an acid-soluble collagen in solution was measured, so that the pH largely increased in the temperature region including ca. 40 degrees C of the thermal denaturation temperature. As the changes of the optical rotation, alpha(D), and the electrical conductivity, sigma, were observed in this temperature region, the change of the pH is due to the thermal denaturation of collagen. The differential curve of pH vs. t gave clear deflection points and a large peak at ca. 40 degrees C. The thermal denaturation temperature, t(d), could be estimated from the peak temperature. The t(d) obtained by the measurement of the alpha(D), t(d alpha D), decreased with the decrease in the heating rate and with the increase in the concentration of collagen. However, the t(d) obtained through the measurement of the pH, t(dpH), was independent of the variations of the heating rate and the concentration of collagen. These measurements were carried out for different kinds of collagens prepared by various methods. Some of them had one kind of peak; others had two kinds of peaks. The t(dpH) correlated with the t(d alpha D) for different kinds of collagen preparations. Therefore, the measurement of the temperature dependence of the pH was useful for the determination of the t(d).