Clearly you're not succeeding in finding the magic formula that invokes serial processing. This blog isn't the best vehicle for providing technical support: please use the saxon-help mailing list or forum on sourceforge to explain exactly what you are doing in the stylesheet, and then there's a chance I can identify what you are doing wrong. It is *very* sensitive to slight variations in the XPath expressions that you use. For 4Gb, you're not going to succeed with an in-memory transformation, so allocating more memory won't help; you need to find the formula for serial processing, in which case you need far less memory than you are giving it.