SUMMARYmCrypton is a 64‐bit lightweight block cipher designed for use in low‐cost and resource‐constrained applications such as RFID tags and sensors in wireless sensor networks. In this paper, we investigate the strength of this cipher against related‐key impossible differential cryptanalysis. First, we construct two 6‐round related‐key impossible differentials for mCrypton‐96 and mCrypton‐128. Then, using these distinguishers, we present 9‐round related‐key impossible differential attacks on these two versions. The attack on mCrypton‐96 requires 259.9 chosen plaintexts, and has a time complexity of about 274.9 encryptions. The data and time complexities for the attack on mCrypton‐128 are 259.7 chosen plaintexts and 266.7 encryptions, respectively. Copyright © 2011 John Wiley & Sons, Ltd.