This paper presents a symmetric key establishment protocol, called Event-Based Reconciliation (ERS), that does not require sensor nodes to store pairwise keys prior to deployment. The novel idea behind ERS is to establish symmetric keys by exchanging the secret key information via distributed source coding that uses some event data as side information. Any two neighboring sensor nodes that are able to detect the same event first exchange some key information using distributed source coding and data of the event detected in the overlapping sensing region, and reconcile a symmetric key in two iterations. A key expansion algorithm is also Presented to enhance the recovery of key bits in. the presence of bit errors and to improve the secrecy of the key establishment. Performance evaluation results show that ERS has low memory and communication overhead compared to previous symmetric key distribution schemes.