To be considered beneficial, nutrient-rich, and be applied safely to soil, a batch of compost should be assessed for its maturity. We tested a new method to follow the time course of maturation on a compost consisting of sheep manure, grape and coffee by-products. This method was based upon the content of the humic acid-like compost fraction in methoxyl (-OCH3) groups, functional groups characteristic of lignin structural units. It involved the conversion of OCH3 groups to the corresponding alkyl iodide (ICH3) groups by treatment with boiling hydriodic acid, before gas chromatography determination. Among the various characteristics studied (ash, organic C, N, lignin), the OCH3 content appeared to be a good tracer of compost maturity.