Abstract An automated measuring system was developed for monitoring the concentration of individual chemical ions in nutrient film solutions. This used ion-selective electrodes to measure pH, nitrate, potassium, calcium, sodium and chloride in solutions used for growing greenhouse tomatoes. The use of frequent and regular calibrations with two solutions which encompassed the concentrations existing in the nutrient solution enabled both short and long term drift of the electrodes to be accommodated, and electrode life to be extended. The accuracy of pH measurement was shown to be ± 0·4 pH units, while the concentrations of nitrate and potassium could be obtained with uncertainties of ± 10 and ± 20% respectively. The repeatability of measurements made on the same samples were within 10 p.p.m. or 0·05 pH units, with the exception of a value of 27 p.p.m. obtained with calcium. The life of the glass and solid state electrodes used for pH, sodium and chloride exceeded 2 years, but that of the p.v.c. electrodes used for nitrate, potassium and calcium, were typically 4, 2 and less than one month, respectively.