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Tier 4 Final legislation commences from 2008 - 2015, depending on engine power. At the same time the use of biodiesel is being incentivized or mandated in many countries. This is driving up the proportion of biodiesel available to the diesel engine fleet and so it is important to understand its impact on possible Tier 4 Final engine and aftertreatment systems. One of the solutions being explored to meet Tier 4 Final emissions regulations is selective catalytic reduction (SCR) using urea and an appropriate catalyst. Previous researchers have highlighted the potential for biodiesel to have a much greater impact on percentage increase in tailpipe NOx on engines equipped with Urea SCR aftertreatment than has historically been the case for engine-out NOx increase. Increases of as much as 80% have been presented, but without knowledge of the engine-out or absolute NOx emission data, it has not been possible to draw any conclusions from some of these publications.