Whether pH is being used to test a sample against a legal requirement or specification; as part of an analytical method; for monitoring and controlling a reaction; as a process control in the chemical industry; or for the environmental monitoring of waste and effluents, it is important that all pH measurements are carried out in a logical and consistent manner, paying careful attention to experimental procedures, in order to obtain reliable results. This guide provides scientists with the kwledge of how to do just that, first by outlining the principles of pH measurement and buffer solutions. pH meters and electrodes are then discussed, including selection criteria and the care of electrodes. Finally, sections on making pH measurements and uncertainty are followed by a set of practical exercises. Measurement of pH is one of the Practical Laboratory Skills Training Guides , a series that aims to make achieving best practice easy. These manuals will enable both experienced and inexperienced staff to get the essential basics of any experiment right simply by following the clear and easy to use instructions provided. The guides are written by experienced scientists and include minimal theory, plenty of practical exercises in order to assess competence, and trouble-shooting information.