||In this study the velocity of 12 Altmetric.com data sources in disseminating newly published research outputs is investigated. The Velocity Index is proposed to make a comparison of velocity among Altmetric.com data sources across document types and subject fields. Some altmetric posts accumulated very fast within the first few days after publication, such as Reddit, Twitter, News, and Facebook, while posts of Policy documents, Wikipedia, Q&A, and Peer review with low Velocity Index values accrued relatively slowly. Most data sources’ velocity degree also change by document types and subject fields. The velocity of most data sources confronted with the type of Review is lower than the overall and Article, while Editorial Material and Letter are higher. In general, most altmetric data sources show higher velocity values in the fields of Multidisciplinary Journals and Natural Sciences.