Viscosity data for ionic liquids (ILs) are needed for the theoretical study on viscosity or for the design/development of industrial process that involves ILs; understanding the relationship between ionic structure and viscosity is also desired to more rationally design and synthesize ILs with ideal viscosity. A database for the viscosity of pure ILs and their binary/ternary mixtures with molecular compounds is created by performing a comprehensive collection from published scientific literature sources worldwide covering the period from 1970 to 2009. In this database, there are 5046 data entries, 696 ILs, 306 cations, and 138 anions. Following the database, a direct observation of the effects of ionic structure along with temperature, pressure, and impurity on the viscosity is summarized, and a quantitative structure-property relationship (QSPR) correlation is performed to understand the viscosity at a micro-electronic or molecular level. Through direct observation and QSPR, the relationship between ILs structure and viscosity is addressed. (c) 2011 American Institute of Chemical Engineers AIChE J, 2012