Techniques for acknowledging data transmissions in a multi-carrier wireless communication network are disclosed. In one aspect, a UE determines a number of acknowledgement/negative acknowledgement (ACK/NACK) bits for a data transmission on one more component carriers (CCs) based on information obtained from a grant. The grant may be a downlink grant or an uplink grant, and the information obtained may include a number of CCs scheduled for data transmission and/or identifiers of the scheduled CCs. The UE may determine the number of ACK/NACK bits for acknowledging the data transmission based on the number of scheduled CCs and the identifier of each scheduled CC.