Summary. Critical potassium (K) concentrations for the diagnosis of K deficiency were determined in various shoot parts of faba bean (Vicia faba L. cv. Fiord) and chickpea (Cicer arietinum L. T1587) plants grown at K rates of 0–240 mg K/kg in a K-deficient soil in the glasshouse. Shoot growth of both species increased with rates of K applied and with plant age but faba bean was more responsive to K application than chickpea. Potassium concentrations in plant parts greatly increased with increasing K supply. In faba bean, K concentrations in all shoot parts decreased with plant age irrespective of K supply while in chickpea K concentrations in leaves of K-deficient plants increased with plant age but those in stems decreased. For both species, young leaves had higher K concentrations than old tissues, especially in K-deficient plants. A greater proportion of leaf K was stored in the petioles when K supply was adequate than when K was deficient. Critical K concentrations were determined using the hand-fitted curve and Mitscherlich model at 90% maximum shoot weight and the 2-phase linear model. The critical concentrations varied between plant parts and among the methods used. Critical values decreased with leaf age and were higher in leaf petioles than in leaf blades. Critical K concentrations, developed from hand-fitted curves, in the 1st plus 2nd and 3rd plus 4th leaf blades of faba bean, and leaf petioles below the youngest fully expanded leaf (YFEL) of chickpea were similar between days 48 and 73 for faba bean and between days 55 and 80 for chickpea. It is recommended that the critical values for the diagnosis of K deficiency at 7–8-leaf stages are 1.3–1.5% in YFEL, 1.1–1.2% in the 1st plus 2nd leaf blades below the YFEL and 1.8–2.0% in whole shoot of faba bean, and 1.4–1.5% in YFEL, 2.7–2.8% in the 1st plus 2nd leaf petioles and 2.1–2.2% in whole shoot of chickpea. The gradient of K concentration between young and old leaves occurred before a decrease in shoot growth and may be used for the prognosis of K deficiency.