Potentiometric gas electrodes are suitable to investigate the kinetic properties and activi-ties of enzymes. Relative activities and some properties of glutamate decarboxylase were studied in squash family using a pCO2 membrane electrode. Crude plant honıogenates were used in or- der to avoid time çonsuming purification steps and possible conformational changes. L-Gluta- mate quantities in the crude extracts were estimated from V-[S] curves by extrapolation. No direct correlation between these guantities and enzyme activities was observed. Rather simsar values of Km were obtained for squash family which are in the same order of magnitude as those of other higher plants.