EXI is a very compact representation for the Extensible Markup Language (XML) Information Set that is intended to simultaneously optimize performance and the utilization of computational resources. The EXI format uses a hybrid approach drawn from the information and formal language theories, plus practical techniques verified by measurements, for entropy encoding XML information. Using a relatively simple algorithm, which is amenable to fast and compact implementation, and a small set of datatype representations, it reliably produces efficient encodings of XML event streams.

The EXI format is derived from the AgileDelta Efficient XML format. EXI was chosen as W3C's Binary XML format after an evaluation of various proposals that included Fast Infoset.

An advantage of EXI over Fast Infoset is that EXI (optionally) uses more constraints from the XML schema. This can make the EXI data more compact; for example, if the XML schema specifies that elements named 'bar' may only exist within elements named 'foo', EXI can assign a shorter token to the 'bar' element, knowing that it doesn't have to share the same token space as elements that occur elsewhere in the document.

The main disadvantage is that to take advantage of this "schema-informed" compression, not only does the document require a schema, but the decoder needs a copy of the same schema that the encoder used.