Factors governing pH changes that samples taken from systems closed to the atmosphere (pipe corrosion loops, distribution mains, household plumbing, ground and hypolimnotic waters) may undergo before and during conventional pH measurement is briefly discussed. Noticeable pH changes attributable to sample measurement while open to a different carbon dioxide partial pressure than in the source location were observed. A procedure and apparatus is outlined to minimize sample contact with the atmosphere during pH measurement, resulting in greater accuracy and reproducibility. Several other procedures to improve pH accuracy relating to electrode calibration and buffer solution storage are discussed.