We report on a replicated spaced-plant trial in Wales of 244 diverse Miscanthus accessions grouped by classical taxonomy into M. sinensis, M. sacchariflorus and M. x giganteus. Large differences in cell wall composition were detected between taxonomic groups. Significantly higher concentrations of NDF (88.19, 85.59 and 85.03 %DW; S.E.D. of 0.300; P < 0.001); ADL (11.76, 10.15 and 9.07 %DW; S.E.D. of 0.087; P < 0.001) and cellulose (47.09, 42.94 and 42.50 %DW; S.E.D. of 0.255; P < 0.001); and significantly lower concentrations of hemicellulose (29.43, 32.50 and 33.46 %DW; S.E.D. of 0.130; P < 0.001) were present in the samples from M. x giganteus compared to the other groups. Growth year and the interaction between taxonomic group and growth year had smaller significant effects. Strong correlations were found between ADL and cellulose (R = 0.72), and hemicellulose (R = −0.76) in the M. x giganteus samples but correlations were weaker for M. sacchariflorus (R = 0.66 and −0.59) and M. sinensis (R = 0.46 and −0.40). Eleven genotypes had been assessed previously in the U.K. during the European Miscanthus Improvement (EMI) programme. These generally had higher concentrations of ADL and hemicellulose, and lower concentrations of cellulose than were reported for the U.K. site during the EMI study. These results suggest that cell wall composition and ADL concentration in particular, is highly stable between growth years but is affected by site location. Furthermore, there are differences in cell wall composition between Miscanthus taxonomic groups that may prove valuable for breeding new varieties that are compositionally matched to conversion process.