The effects of potassium nutrition [0, 6.25, 12.50, 25.00 g(K) m-2 of K2SO4 or KCl] on gas exchange characteristics and water relations in four cultivars (CIM-448, CIM-1100, Karishma, S-12) of cotton were assessed under an arid environment. Net photosynthetic rate (PN) and transpiration rate (E) increased with increased K supply. The leaf pressure potential (Ψp) increased significantly by the addition of 25.00 g(K) m-2 compared to zero K level. The water use efficiency (PN/E) was improved by 24.6 % under the highest K dose compared to zero K. There were positive correlations (0.99**, 0.98**, 0.95**, 0.97**) between K-doses and PN, E, Ψp, and PN/E, respectively. and H. Pervez, M. Ashraf, M. I. Makhdum.