Used frying oils (UFO) are an interesting and important source material for manufacturing of methyl esters (ME) as alternative fuels. The technology of methyl ester preparation from UFO (abbreviated as UFOME) is similar as in the case of fresh oils. However, in spite of a high degree of acylglycerol conversion into methyl esters, UFOME frequently exhibit insufficient methyl ester content, increased viscosity and density, reduced oxidation stability, increased carbon residue value, etc. Low ester content is related especially to the formation and presence of oligomeric products that result from reactions involving double bonds in acyls. A method suitable for studying changes in UFO and UFOME composition is NIR/MID spectroscopy, particularly bands at 2880 or 2930 nm. Out of 31 UFOME samples subject to evaluation, 16 samples with insufficient ester content of less than 96.5% had absorbance values above 1.80 at 2880 nm. On the other hand, all the remaining 15 samples with sufficient ester content above 96.5% after transesterification had absorbance values up to 1.80 at 2880 nm. Distillation treatment of UFOME samples with insufficient ester content gave rise to UFOME having ester content in line with the EN 14 214 standard. While distillation can be used to treat UFOME in order to obtain standard quality, no treatment at the UFO level is feasible for the time being. UFO extraction with supercritical CO2 can be applied to achieve monomeric and oligomeric UFO separation, but the separation yield and grade are low and investment and operating costs excessively high. (C) 2013 Elsevier Ltd. All rights reserved.