The decomposition in soil of plant material from the stems of uniformly grown tobacco plants with genetic modifications to lignin biosynthesis was investigated by measuring CO2 production over 74 days. The effect on decomposition of readily available C, N and combined C plus N (as glucose, NH4NO3 and alanine, respectively) were determined by adding these compounds to the soil before the addition of plant materials. The genetic modifications to tobacco were the introduction of partial or antisense transgenes for cinnamyl alcohol dehydrogenase (CAD), caffeic acid O-methyl transferase (COMT) and cinnamoyl CoA reductase (CCR), which are all enzymes that catalyse key steps in the monolignol pathway. In line with previous reports, reduced CCR material decomposed significantly faster than the reduced CAD or reduced COMT materials, which decomposed significantly faster than the unmodified material. All the plant materials had large C-to-N ratios (58, 72, 61 and 52 for the unmodified, reduced CAD, reduced COMT and reduced CCR materials, respectively), but the relationships between C-to-N ratio and decomposition rate were not consistent across all materials. Addition of glucose slowed the decomposition rate of all the materials. This is consistent with the additional C leading to a surplus of C relative to other nutrients at the sites of plant residue decomposition, but the reduction in decomposition rate was directly related to the combined C-to-N ratio of the plant material and soil amendment. In the case of reduced CCR material, addition of N as either NH 4NO3 or alanine led to an increase in decomposition rate, indicating that a constraint on decay of this material was alleviated by N addition. However, addition of N did not increase the decomposition of the unmodified, reduced CAD and reduced COMT materials, which indicates that their decomposition was constrained by factors other than N. We have shown significant effects of targeted lignin modification on decomposition in laboratory studies over a relatively short period (< 74 days). However, longer-term and more detailed field-based investigations are needed before the wider ecological significance of the differences in decomposition can be properly assessed.