The BMW 525tds Biodiesel was a version of the BMW 525tds modified to run on the biofuel rapeseed methyl ester (RME). Since biofuel has the negative characteristic of attacking and dissolving seals and fuel hoses, these were made of the material fluororubber in the model version approved for biodiesel. As well as being exempt from fuel duty for mineral oil, running on biodiesel offered the advantage of reducing levels of carbon-monoxide, hydrocarbon, particle and soot emissions. This benefit contrasted with a slightly increased emission of nitrogen oxides (NOx).