I hadn’t come across this one before, but buried in support article 918643 about “How to troubleshoot a memory leak or an out-of-memory exception in the BizTalk Server process”, is the fact that some BizTalk mapper functiods leak memory. How much memory is dependent on the size of the assembly that contains the maps that use the leaky functiods. The leaky functiods are all the ones that are actually implemented using inline script in XSLT. The support article has a useful table that highlights which ones these are: Functoids Inline script? All String Functoids Yes All Mathematical Functoids Yes All Logical Functoids except IsNil Yes Logical IsNil Functoid No All Date/Time Functoids Yes All Conversion Functoids Yes All Scientific Functoids Yes All Cumulative Functoids Yes All Database Functoids No   Advanced Functoids Inline script? Looping Functoid No Value Mapping Flattening Functoid No Assert Functoid No Table Extractor Functoid No Table ...