The solvent extraction of alkanes from faeces and herbage samples at two different temperatures (cold: 15-25 degrees C and hot: 65 degrees C) was studied in four samples of different matrix types (cattle faeces, sheep faeces, hill grass and heather), in two experiments performed at Villaviciosa, Asturias, Spain in 1994. Two internal standards (IS) of different chain length (C-22 and C-34) were used to estimate alkane concentrations. Significant differences were detected in alkane extraction derived from temperature of extraction, IS and sample matrix. At the cold temperature, long-chain alkane extraction was not complete, resulting in errors in the estimation of alkane concentration when a long-chain alkane (C-34) was used as the only internal standard. However, under hot extraction, long-chain alkanes were completely extracted by the heptane, although estimates made with C-22 or C-34 as IS were not identical. These results suggest that it would be appropriate to use two internal standards with short and long carbon chain, such as C-22 and C-34, in routine analyses to establish the completeness of alkane extraction, even under hot conditions, by calculating the relative ratio of both IS in extracts compared to the original C-22:C-34 ratio added to the samples. Any increase or decrease in expected peak areas could be adjusted for all the alkanes in the extracts, and the accuracy of alkane concentration measurements (and therefore the reliability of estimates of intake and especially of diet selection) would be improved.