Potentiometric pH measurement in low ionic strength stream waters is difficult due to high system variability and low ionic activity. Imprecise measurement may result from equipment response in such waters and solution pH drift caused by gaseous exchange and temperature change. An acceptable method for field pH determination must utilize sensitive equipment and stringent measurement stability criteria to ensure accurate and reliable measurements. Four methods for in situ and streamside pH determination were used to collect data from three low-ionic strength streams. The methods are compared, using analysis of variance and subjective evaluation.