Pyrolysis oil has been identified as a possible alternative fuel source; however, widespread use is hindered by-high acidity and water content. These negative characteristics can be mitigated by blending with, for example, biodiesel, marine gas oil, and butanol. These blended Samples, can he unstable and often separate into two distinct phases. NMR spectroscopy is a well-established spectroscopic technique that is finding increasing application in the analysis of pyrolysis oil and blended fuels derived from it. Here, slice selective NMR, where the NMR spectrum of only a thin slice of the total sample is acquired, is used to study, noninvasively, how the constituent components of blended biofuel samples are partitioned between the two layers. Understanding the outcome of the-phase separation is an important step toward understanding why the blended oil samples separate and may provide answers to mitigating and-eventually solving the problem. The NMR method was successfully used to analyze a umber of separated biofuel samples, typically separated into an oil layer, containing marine gas Oil and biodiesel, above a bio-oil layer with a high water and butanol content.