This dataset contains identifiers for 8,410,431 tweets that were collected between September 19, 2017 and October 5, 2017 that mentioned #CatalanReferendum, #CatalalonianReferendum, #Catalonia, #1oct, #1o or #votarem. These hashtags were used in the lead up to the Catalan Independence Referendum on October 1, 2017. The referendum was declared illegal under Spanish law, and the Spanish police attempted to prevent it. The data collection was a collaboration with Vicenç Ruiz Gómez and Aniol Maria of the Society of CatalanArchivists working in conjunction with Ed Summers of the Maryland Institute for Technology in the Humanities. The hashtags were selected after monitoring the #CatalanReferendum hashtag for several hours on September 28 to determine what the top hashtags being used were. The tweets themselves were collected from the Twitter Search API using twarc and its twarc-archive utility. twarc-archive was run every hour to collect the tweets that occurred since the last run.