In this study, we analyzed KERIS Document Delivery Service (DDS) using its transaction data for the period of nine years from 2004 to 2012. We first examined the overall statistics focusing on member contributions, and conducted a network analysis based on the records of request/response (supply) between member libraries. Key findings include the following: First, in over 80% of member libraries, the number of outgoing requests exceeded the number of their responses to incoming requests. That is, for the vast majority of member libraries, their participation was concentrated on the request side. Second, KERIS DDS relies heavily on a relatively small number of top contributors, especially on the supply side. While the top contributors were active in both requests and responses (supplies), in most cases, they received and processed a disproportionally large number of requests. Third, the network analysis based on DDS requests for journal articles in 2012 further revealed the central role of top contributors. The level and pattern of concentration, however, appeared to differ by subjects (DDC). Three main patterns of centralization were found in different subjects - a network centered on a single member, a network having multiple centers, or a distributed network.