White matter pathology is characterized by demyelination and axonal loss. Diffusion MRI and relaxometry cannot separate these processes because they are primarily sensitive to axons and myelin, respectively. We used diffusion-T1-T2-relaxation MRI with tensor-valued encoding to support a twelve-parameter microstructure model including myelin. Results yielded brain parameter maps and values of the axonal and myelin volume fractions and the g-ratio that were in accordance with previous results from histology. The proposed approach can theoretically disambiguate between demyelination and axonal loss and could thus become a valuable tool for assessing disease severity and treatment response in pathologies that affect white matter.