The content of carbonyl groups in a lignin sample is given by the sum of aldehydric, ketonic, and quinonoid groups. This data is fundamental for both technical and analytical-grade lignins. On the one hand, it is of crucial relevance for technical lignins when planning upgrade processes involving its hydrophobicity, self-assembly behaviour, and tendency to (de)polymerise. On the other hand, it is vital to better understand the structure of analytical-grade lignins, allowing for an in-depth investigation of their biogenesis.In the present contribution, a simple, reliable, and robust methodology for determining carbonyl group content in lignin is presented. This approach involves the use of 31 P NMR (after phosphitylation with 2-chloro-4,4’,5,5’-tetramethyl-1,3,2-dioxaphospholane) coupled with a quantitative and selective reduction of carbonyl to hydroxyl groups. The carbonyl groups content is then calculated as the difference in aliphatic and phenolic hydroxyl group content before and after reduction.The reduction conditions have been optimised for both technical (kraft and organosolv lignins) and analytical-grade (p-dioxane and enzymatically mild acidolysis lignins) lignin preparations, resultingin a general, semi-micro scale (150 mg of sample are required) protocol suitable for all of them.The results obtained for the quantitation of carbonyl groups in different lignins (technical- and analytical-grade) are consistent with those obtained using quantitative 13 C NMR and the standard oximation method. The experimental error aligns with that typical for 31 P NMR analyses (< 8%).In addition, the nature of the quantified carbonyl groups was elucidated by comparing the HSQC spectra of the lignins before and after the reduction. The most abundant types of carbonyl groupsare represented by vanillyl- and syringyl-aldehydes, β-aryl-propiophenones, and Hibbert-ketones.The present methodology is suitable for both academia and industry, where low field bench-top instruments capable to perform hydroxyl groups analyses of lignin via 31 P NMR analyses are becoming widely popular.